Advanced PHP Programming- P2

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  1. 28 Chapter 1 Coding Styles Compare this with the following: NamePosition The second code fragment is cleaner and does not obfuscate the HTML by unnecessari- ly using echo. As a note, using the syntax, which is identical to , requires the use of short_tags, which there are good reasons to avoid. print Versus echo print and echo are aliases for each other; that is, internal to the engine, they are indistinguishable. You should pick one and use it consistently to make your code easier to read. Using Parentheses Judiciously You should use parentheses to add clarity to code.You can write this: if($month == ‘february’) { if($year % 4 == 0 && $year % 100 || $year % 400 == 0) { $days_in_month = 29; } else { $days_in_month = 28; } } However, this forces the reader to remember the order of operator precedence in order to follow how the expression is computed. In the following example, parentheses are used to visually reinforce operator precedence so that the logic is easy to follow: if($month == ‘february’) { if((($year % 4 == 0 )&& ($year % 100)) || ($year % 400 == 0)) { $days_in_month = 29; } else { $days_in_month = 28; } } You should not go overboard with parentheses, however. Consider this example: if($month == ‘february’) { if(((($year % 4) == 0 )&& (($year % 100) != 0)) || (($year % 400) == 0 )) { $days_in_month = 29;
  2. Documentation 29 } else { $days_in_month = 28; } } This expression is overburdened with parentheses, and it is just as difficult to decipher the intention of the code as is the example that relies on operator precedence alone. Documentation Documentation is inherently important in writing quality code. Although well-written code is largely self-documenting, a programmer must still read the code in order to understand its function. In my company, code produced for clients is not considered complete until its entire external application programming interface (API) and any inter- nal idiosyncrasies are fully documented. Documentation can be broken down into two major categories: n Inline comments that explain the logic flow of the code, aimed principally at peo- ple modifying, enhancing, or debugging the code. n API documentation for users who want to use the function or class without read- ing the code itself. The following sections describe these two types of documentation. Inline Comments For inline code comments, PHP supports three syntaxes: nC-style comments—With this type of comment, everything between /* and */ is considered a comment. Here’s an example of a C-style comment: /* This is a c-style comment * (continued) */ n C++-style comments—With this type of comment, everything on a line fol- lowing // is considered a comment. Here’s an example of a C++-style comment: // This is a c++-style comment n Shell/Perl-style comments—With this type of comment, the pound sign (#) is the comment delimiter. Here’s an example of a Shell/Perl-style comment: # This is a shell-style comment In practice, I avoid using Shell/Perl-style comments entirely. I use C-style comments for large comment blocks and C++-style comments for single-line comments.
  3. 30 Chapter 1 Coding Styles Comments should always be used to clarify code.This is a classic example of a worth- less comment: // increment i i++; This comment simply reiterates what the operator does (which should be obvious to anyone reading the code) without lending any useful insight into why it is being per- formed.Vacuous comments only clutter the code. In the following example, the comment adds value: // Use the bitwise “AND” operatorest to see if the first bit in $i is set // to determine if $i is odd/even if($i & 1) { return true; } It explains that we are checking to see whether the first bit is set because if it is, the number is odd. API Documentation Documenting an API for external users is different from documenting code inline. In API documentation, the goal is to ensure that developers don’t have to look at the code at all to understand how it is to be used. API documentation is essential for PHP libraries that are shipped as part of a product and is extremely useful for documenting libraries that are internal to an engineering team as well. These are the basic goals of API documentation: n It should provide an introduction to the package or library so that end users can quickly decide whether it is relevant to their tasks. n It should provide a complete listing of all public classes and functions, and it should describe both input and output parameters. n It should provide a tutorial or usage examples to demonstrate explicitly how the code should be used. In addition, it is often useful to provide the following to end users: n Documentation of protected methods n Examples of how to extend a class to add functionality Finally, an API documentation system should provide the following features to a devel- oper who is writing the code that is being documented: n Documentation should be inline with code.This is useful for keeping documenta- tion up-to-date, and it ensures that the documentation is always present.
  4. Documentation 31 n The documentation system should have an easy and convenient syntax.Writing documentation is seldom fun, so making it as easy as possible helps ensure that it gets done. n There should be a system for generating beautified documentation.This means that the documentation should be easily rendered in a professional and easy-to- read format. You could opt to build your own system for managing API documentation, or you could use an existing package. A central theme throughout this book is learning to make good decisions regarding when it’s a good idea to reinvent the wheel. In the case of inline documentation, the phpDocumentor project has done an excellent job of creating a tool that satisfies all our requirements, so there is little reason to look elsewhere. phpDocumentor is heavily inspired by JavaDoc, the automatic documentation system for Java. Using phpDocumentor phpDocumentor works by parsing special comments in code.The comment blocks all take this form: /** * Short Description * * Long Description * @tags */ Short Description is a short (one-line) summary of the item described by the block. Long Description is an arbitrarily verbose text block. Long Description allows for HTML in the comments for specific formatting. tags is a list of phpDocumentor tags. The following are some important phpDocumentor tags: Tag Description @package [package name] The package name @author [author name] The author information @var [type] The type for the var statement following the comment @param [type [description]] The type for the input parameters for the function following the block @return [type [description]] The type for the output of the function You start the documentation by creating a header block for the file: /** * This is an example page summary block *
  5. 32 Chapter 1 Coding Styles * This is a longer description where we can * list information in more detail. * @package Primes * @author George Schlossnagle */ This block should explain what the file is being used for, and it should set @package for the file. Unless @package is overridden in an individual class or function, it will be inherited by any other phpDocumentor blocks in the file. Next, you write some documentation for a function. phpDocumentor tries its best to be smart, but it needs some help. A function’s or class’s documentation comment must immediately precede its declaration; otherwise, it will be applied to the intervening code instead. Note that the following example specifies @param for the one input parameter for the function, as well as @return to detail what the function returns: /** * Determines whether a number is prime (stupidly) * * Determines whether a number is prime or not in * about the slowest way possible. * * for($i=0; $i
  6. Documentation 33 Figure 1.3 phpdoc output for primes.php. For a slightly more complicated example, look at this basic Employee class:
  7. 34 Chapter 1 Coding Styles */ var $salary; /** * @var number */ var $employee_id; /** * The class constructor * @param number */ function Employee($employee_id = false) { if($employee_id) { $this->employee_id = $employee_id; $this->_fetchInfo(); } } /** * Fetches info for employee * * @access private */ function _fetchInfo() { $query = “SELECT name, salary FROM employees WHERE employee_id = $this->employee_id”; $result = mysql_query($query); list($this->name, $this->department_id) = mysql_fetch_row($result); } /** * Returns the monthly salary for the employee * @returns number Monthly salary in dollars */ function monthlySalary() { return $this->salary/12; } } ?> Note that _fetchInfo is @access private, which means that it will not be rendered by phpdoc.
  8. Further Reading 35 Figure 1.4 demonstrates that with just a bit of effort, it’s easy to generate extremely pro- fessional documentation. Figure 1.4 The phpdoc rendering for Employee. Further Reading To find out more about phpDocumentor, including directions for availability and installa- tion, go to the project page at www.phpdoc.org. The Java style guide is an interesting read for anyone contemplating creating coding standards.The official style guide is available from Sun at http://java.sun.com/ docs/codeconv/html/CodeConvTOC.doc.html.
  9. 2 Object-Oriented Programming Through Design Patterns B Y FAR THE LARGEST AND MOST HERALDED change in PHP5 is the complete revamp- ing of the object model and the greatly improved support for standard object-oriented (OO) methodologies and techniques.This book is not focused on OO programming techniques, nor is it about design patterns.There are a number of excellent texts on both subjects (a list of suggested reading appears at the end of this chapter). Instead, this chap- ter is an overview of the OO features in PHP5 and of some common design patterns. I have a rather agnostic view toward OO programming in PHP. For many problems, using OO methods is like using a hammer to kill a fly.The level of abstraction that they offer is unnecessary to handle simple tasks.The more complex the system, though, the more OO methods become a viable candidate for a solution. I have worked on some large architectures that really benefited from the modular design encouraged by OO techniques. This chapter provides an overview of the advanced OO features now available in PHP. Some of the examples developed here will be used throughout the rest of this book and will hopefully serve as a demonstration that certain problems really benefit from the OO approach. OO programming represents a paradigm shift from procedural programming, which is the traditional technique for PHP programmers. In procedural programming, you have data (stored in variables) that you pass to functions, which perform operations on the data and may modify it or create new data. A procedural program is traditionally a list of instructions that are followed in order, using control flow statements, functions, and so on.The following is an example of procedural code:
  10. 38 Chapter 2 Object-Oriented Programming Through Design Patterns function goodbye($name) { return “Goodbye $name!\n”; } function age($birthday) { $ts = strtotime($birthday); if($ts === -1) { return “Unknown”; } else { $diff = time() - $ts; return floor($diff/(24*60*60*365)); } } $name = “george”; $bday = “10 Oct 1973”; echo hello($name); echo “You are “.age($bday).” years old.\n”; echo goodbye($name); ? > Introduction to OO Programming It is important to note that in procedural programming, the functions and the data are separated from one another. In OO programming, data and the functions to manipulate the data are tied together in objects. Objects contain both data (called attributes or proper- ties) and functions to manipulate that data (called methods). An object is defined by the class of which it is an instance. A class defines the attrib- utes that an object has, as well as the methods it may employ.You create an object by instantiating a class. Instantiation creates a new object, initializes all its attributes, and calls its constructor, which is a function that performs any setup operations. A class constructor in PHP5 should be named _ _constructor() so that the engine knows how to iden- tify it.The following example creates a simple class named User, instantiates it, and calls its two methods:
  11. Introduction to OO Programming 39 return “Hello $this->name!\n”; } public function goodbye() { return “Goodbye $this->name!\n”; } public function age() { $ts = strtotime($this->birthday); if($ts === -1) { return “Unknown”; } else { $diff = time() - $ts; return floor($diff/(24*60*60*365)) ; } } } $user = new User(‘george’, ‘10 Oct 1973’); echo $user->hello(); echo “You are “.$user->age().” years old.\n”; echo $user->goodbye(); ?> Running this causes the following to appear: Hello george! You are 29 years old. Goodbye george! The constructor in this example is extremely basic; it only initializes two attributes, name and birthday.The methods are also simple. Notice that $this is automatically created inside the class methods, and it represents the User object.To access a property or method, you use the -> notation. On the surface, an object doesn’t seem too different from an associative array and a collection of functions that act on it.There are some important additional properties, though, as described in the following sections: n Inheritance—Inheritance is the ability to derive new classes from existing ones and inherit or override their attributes and methods. n Encapsulation—Encapsulation is the ability to hide data from users of the class. n Special Methods—As shown earlier in this section, classes allow for constructors that can perform setup work (such as initializing attributes) whenever a new object is created.They have other event callbacks that are triggered on other common events as well: on copy, on destruction, and so on.
  12. 40 Chapter 2 Object-Oriented Programming Through Design Patterns n Polymorphism—When two classes implement the same external methods, they should be able to be used interchangeably in functions. Because fully understand- ing polymorphism requires a larger knowledge base than you currently have, we’ll put off discussion of it until later in this chapter, in the section “Polymorphism.” Inheritance You use inheritance when you want to create a new class that has properties or behav- iors similar to those of an existing class.To provide inheritance, PHP supports the ability for a class to extend an existing class.When you extend a class, the new class inherits all the properties and methods of the parent (with a couple exceptions, as described later in this chapter).You can both add new methods and properties and override the exiting ones. An inheritance relationship is defined with the word extends. Let’s extend User to make a new class representing users with administrative privileges.We will augment the class by selecting the user’s password from an NDBM file and providing a compari- son function to compare the user’s password with the password the user supplies: class AdminUser extends User{ public $password; public function _ _construct($name, $birthday) { parent::_ _construct($name, $birthday); $db = dba_popen(“/data/etc/auth.pw”, “r”, “ndbm”); $this->password = dba_fetch($db, $name); dba_close($db); } public function authenticate($suppliedPassword) { if($this->password === $suppliedPassword) { return true; } else { return false; } } } Although it is quite short, AdminUser automatically inherits all the methods from User, so you can call hello(), goodbye(), and age(). Notice that you must manual- ly call the constructor of the parent class as parent::_ _constructor(); PHP5 does not automatically call parent constructors. parent is as keyword that resolves to a class’s parent class.
  13. Introduction to OO Programming 41 Encapsulation Users coming from a procedural language or PHP4 might wonder what all the public stuff floating around is.Version 5 of PHP provides data-hiding capabilities with public, protected, and private data attributes and methods.These are commonly referred to as PPP (for public, protected, private) and carry the standard semantics: n Public—A public variable or method can be accessed directly by any user of the class. n Protected—A protected variable or method cannot be accessed by users of the class but can be accessed inside a subclass that inherits from the class. n Private—A private variable or method can only be accessed internally from the class in which it is defined.This means that a private variable or method cannot be called from a child that extends the class. Encapsulation allows you to define a public interface that regulates the ways in which users can interact with a class.You can refactor, or alter, methods that aren’t public, with- out worrying about breaking code that depends on the class.You can refactor private methods with impunity.The refactoring of protected methods requires more care, to avoid breaking the classes’ subclasses. Encapsulation is not necessary in PHP (if it is omitted, methods and properties are assumed to be public), but it should be used when possible. Even in a single-programmer environment, and especially in team environments, the temptation to avoid the public interface of an object and take a shortcut by using supposedly internal methods is very high.This quickly leads to unmaintainable code, though, because instead of a simple public interface having to be consistent, all the methods in a class are unable to be refac- tored for fear of causing a bug in a class that uses that method. Using PPP binds you to this agreement and ensures that only public methods are used by external code, regard- less of the temptation to shortcut. Static (or Class) Attributes and Methods In addition, methods and properties in PHP can also be declared static. A static method is bound to a class, rather than an instance of the class (a.k.a., an object). Static methods are called using the syntax ClassName::method(). Inside static methods, $this is not available. A static property is a class variable that is associated with the class, rather than with an instance of the class.This means that when it is changed, its change is reflected in all instances of the class. Static properties are declared with the static keyword and are accessed via the syntax ClassName::$property.The following example illustrates how static properties work: class TestClass { public static $counter; } $counter = TestClass::$counter;
  14. 42 Chapter 2 Object-Oriented Programming Through Design Patterns If you need to access a static property inside a class, you can also use the magic keywords self and parent, which resolve to the current class and the parent of the current class, respectively. Using self and parent allows you to avoid having to explicitly reference the class by name. Here is a simple example that uses a static property to assign a unique integer ID to every instance of the class: class TestClass { public static $counter = 0; public $id; public function _ _construct() { $this->id = self::$counter++; } } Special Methods Classes in PHP reserve certain method names as special callbacks to handle certain events.You have already seen _ _construct(), which is automatically called when an object is instantiated. Five other special callbacks are used by classes: _ _get(), _ _set(), and _ _call() influence the way that class properties and methods are called, and they are covered later in this chapter.The other two are _ _destruct() and _ _clone(). _ _destruct() is the callback for object destruction. Destructors are useful for clos- ing resources (such as file handles or database connections) that a class creates. In PHP, variables are reference counted.When a variable’s reference count drops to 0, the variable is removed from the system by the garbage collector. If this variable is an object, its _ _destruct() method is called. The following small wrapper of the PHP file utilities showcases destructors: class IO { public $fh = false; public function _ _construct($filename, $flags) { $this->fh = fopen($filename, $flags); } public function _ _destruct() { if($this->fh) { fclose($this->fh); } } public function read($length) {
  15. Introduction to OO Programming 43 if($this->fh) { return fread($this->fh, $length); } } /* ... */ } In most cases, creating a destructor is not necessary because PHP cleans up resources at the end of a request. For long-running scripts or scripts that open a large number of files, aggressive resource cleanup is important. In PHP4, objects are all passed by value.This meant that if you performed the follow- ing in PHP4: $obj = new TestClass; $copy = $obj; you would actually create three copies of the class: one in the constructor, one during the assignment of the return value from the constructor to $copy, and one when you assign $copy to $obj.These semantics are completely different from the semantics in most other OO languages, so they have been abandoned in PHP5. In PHP5, when you create an object, you are returned a handle to that object, which is similar in concept to a reference in C++.When you execute the preceding code under PHP5, you only create a single instance of the object; no copies are made. To actually copy an object in PHP5, you need to use the built-in _ _clone() method. In the preceding example, to make $copy an actual copy of $obj (and not just another reference to a single object), you need to do this: $obj = new TestClass; $copy = $obj->_ _clone(); For some classes, the built-in deep-copy _ _clone() method may not be adequate for your needs, so PHP allows you to override it. Inside the _ _clone() method, you not only have $this, which represents the new object, but also $that, which is the object being cloned. For example, in the TestClass class defined previously in this chapter, if you use the default _ _clone() method, you will copy its id property. Instead, you should rewrite the class as follows: class TestClass { public static $counter = 0; public $id; public $other; public function _ _construct() { $this->id = self::$counter++; } public function _ _clone()
  16. 44 Chapter 2 Object-Oriented Programming Through Design Patterns { $this->other = $that->other; $this->id = self::$counter++; } } A Brief Introduction to Design Patterns You have likely heard of design patterns, but you might not know what they are. Design patterns are generalized solutions to classes of problems that software developers encounter frequently. If you’ve programmed for a long time, you have most likely needed to adapt a library to be accessible via an alternative API.You’re not alone.This is a common problem, and although there is not a general solution that solves all such problems, people have recog- nized this type of problem and its varying solutions as being recurrent.The fundamental idea of design patterns is that problems and their corresponding solutions tend to follow repeatable patterns. Design patterns suffer greatly from being overhyped. For years I dismissed design pat- terns without real consideration. My problems were unique and complex, I thought— they would not fit a mold.This was really short-sighted of me. Design patterns provide a vocabulary for identification and classification of problems. In Egyptian mythology, deities and other entities had secret names, and if you could dis- cover those names, you could control the deities’ and entities’ power. Design problems are very similar in nature. If you can discern a problem’s true nature and associate it with a known set of analogous (solved) problems, you are most of the way to solving it. To claim that a single chapter on design patterns is in any way complete would be ridiculous.The following sections explore a few patterns, mainly as a vehicle for show- casing some of the advanced OO techniques available in PHP. The Adaptor Pattern The Adaptor pattern is used to provide access to an object via a specific interface. In a purely OO language, the Adaptor pattern specifically addresses providing an alternative API to an object; but in PHP we most often see this pattern as providing an alternative interface to a set of procedural routines. Providing the ability to interface with a class via a specific API can be helpful for two main reasons: n If multiple classes providing similar services implement the same API, you can switch between them at runtime.This is known as polymorphism.This is derived from Latin: Poly means “many,” and morph means “form.” n A predefined framework for acting on a set of objects may be difficult to change. When incorporating a third-party class that does not comply with the API used by the framework, it is often easiest to use an Adaptor to provide access via the
  17. A Brief Introduction to Design Patterns 45 expected API. The most common use of adaptors in PHP is not for providing an alternative interface to one class via another (because there is a limited amount of commercial PHP code, and open code can have its interface changed directly). PHP has its roots in being a pro- cedural language; therefore, most of the built-in PHP functions are procedural in nature. When functions need to be accessed sequentially (for example, when you’re making a database query, you need to use mysql_pconnect(), mysql_select_db(), mysql_query(), and mysql_fetch()), a resource is commonly used to hold the con- nection data, and you pass that into all your functions.Wrapping this entire process in a class can help hide much of the repetitive work and error handling that need to be done. The idea is to wrap an object interface around the two principal MySQL extension resources: the connection resource and the result resource.The goal is not to write a true abstraction but to simply provide enough wrapper code that you can access all the MySQL extension functions in an OO way and add a bit of additional convenience. Here is a first attempt at such a wrapper class: class DB_Mysql { protected $user; protected $pass; protected $dbhost; protected $dbname; protected $dbh; // Database connection handle public function _ _construct($user, $pass, $dbhost, $dbname) { $this->user = $user; $this->pass = $pass; $this->dbhost = $dbhost; $this->dbname = $dbname; } protected function connect() { $this->dbh = mysql_pconnect($this->dbhost, $this->user, $this->pass); if(!is_resource($this->dbh)) { throw new Exception; } if(!mysql_select_db($this->dbname, $this->dbh)) { throw new Exception; } } public function execute($query) { if(!$this->dbh) { $this->connect(); } $ret = mysql_query($query, $this->dbh); if(!$ret) { throw new Exception;
  18. 46 Chapter 2 Object-Oriented Programming Through Design Patterns } else if(!is_resource($ret)) { return TRUE; } else { $stmt = new DB_MysqlStatement($this->dbh, $query); $stmt->result = $ret; return $stmt; } } } To use this interface, you just create a new DB_Mysql object and instantiate it with the login credentials for the MySQL database you are logging in to (username, password, hostname, and database name): $dbh = new DB_Mysql(“testuser”, “testpass”, “localhost”, “testdb”); $query = “SELECT * FROM users WHERE name = ‘“.mysql_escape_string($name).”‘“; $stmt = $dbh->execute($query); This code returns a DB_MysqlStatement object, which is a wrapper you implement around the MySQL return value resource: class DB_MysqlStatement { protected $result; public $query; protected $dbh; public function _ _construct($dbh, $query) { $this->query = $query; $this->dbh = $dbh; if(!is_resource($dbh)) { throw new Exception(“Not a valid database connection”); } } public function fetch_row() { if(!$this->result) { throw new Exception(“Query not executed”); } return mysql_fetch_row($this->result); } public function fetch_assoc() { return mysql_fetch_assoc($this->result); } public function fetchall_assoc() { $retval = array(); while($row = $this->fetch_assoc()) { $retval[] = $row; } return $retval; } }
  19. A Brief Introduction to Design Patterns 47 To then extract rows from the query as you would by using mysql_fetch_assoc(), you can use this: while($row = $stmt->fetch_assoc()) { // process row } The following are a few things to note about this implementation: n It avoids having to manually call connect() and mysql_select_db(). n It throws exceptions on error. Exceptions are a new feature in PHP5.We won’t discuss them much here, so you can safely ignore them for now, but the second half of Chapter 3, “Error Handling,” is dedicated to that topic. n It has not bought much convenience.You still have to escape all your data, which is annoying, and there is no way to easily reuse queries. To address this third issue, you can augment the interface to allow for the wrapper to automatically escape any data you pass it.The easiest way to accomplish this is by provid- ing an emulation of a prepared query.When you execute a query against a database, the raw SQL you pass in must be parsed into a form that the database understands internally. This step involves a certain amount of overhead, so many database systems attempt to cache these results. A user can prepare a query, which causes the database to parse the query and return some sort of resource that is tied to the parsed query representation. A feature that often goes hand-in-hand with this is bind SQL. Bind SQL allows you to parse a query with placeholders for where your variable data will go.Then you can bind parameters to the parsed version of the query prior to execution. On many database sys- tems (notably Oracle), there is a significant performance benefit to using bind SQL. Versions of MySQL prior to 4.1 do not provide a separate interface for users to pre- pare queries prior to execution or allow bind SQL. For us, though, passing all the vari- able data into the process separately provides a convenient place to intercept the variables and escape them before they are inserted into the query. An interface to the new MySQL 4.1 functionality is provided through Georg Richter’s mysqli extension. To accomplish this, you need to modify DB_Mysql to include a prepare method and DB_MysqlStatement to include bind and execute methods: class DB_Mysql { /* ... */ public function prepare($query) { if(!$this->dbh) { $this->connect(); } return new DB_MysqlStatement($this->dbh, $query); } } class DB_MysqlStatement { public $result;
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