Section 13.5. Other Changes

13.5. Other Changes

Although the compatibility mode covers a few changes between PHP 4 and PHP 5, it does not fix all possible changes. For example, PHP 5 does not allow assigning to $this, which is a problem for a few PEAR classes (at the time of this writing). For example, the Pager/Pager.php file has the following code in its constructor:

 $mode = (isset($options['mode']) ? $options['mode'] : 'Jumping'); $pager_class = 'Pager_' . ucfirst($mode); $pager_classfile = 'Pager' . DIRECTORY_SEPARATOR . $mode . '.php'; require_once $pager_classfile; $this = new $pager_class($options); 

Another PHP 5 change not reverted by compatibility mode is the behavior of get_class().

13.5.1. Assigning to $this

When you use a line in PHP 4 that assigns a value to $this inside a class, depending on an option, a class is selected and an instance to that newly created class is returned. Simplified, the code looks like this (with the offending line in bold):

 <?php     class Jumping {     }     class Sliding {     }     class Pager {         function Pager($type)         {             $this = new $type;         }     }     $pager = new Pager('Jumping'); ?> 

Assigning a new object to $this does not work in PHP 5. When the script runs, it throws the following error:

[View full width]
 Fatal error: Cannot re-assign $this in /book/13-making-the-move/oo assign-to-this.php on  line 11 

The only solution for this problem is to redesign the classes. In this case, an alternative that works with both PHP 4 and PHP 5 is

 <?php     class Pager {         function Pager($options)         {             var_dump($options);         }     }     class Jumping extends Pager {         function Jumping($options)         {             Pager::Pager($options);         }     }     class Sliding extends Pager {         function Jumping($options)         {             Pager::Pager($options);         }     }     $pager = new Jumping('foo'); ?> 

Assigning to $this can also be used to "emulate" an exception, which is necessary because you cannot return errors from a constructor. For example, the Net_Curl PEAR package has the following in its constructor:

 function Net_Curl() {     ...     $ch = curl_init();     if (!$ch) {         $this = new PEAR_Error("Couldn't initialize a new curl handle");     }     ... } 

This is used to emulate an exception. In PHP 5, the correct way would be to use an . . . exception. For this to work, the PEAR_Error class needs to extend the internal PHP Exception class. In the examples here, we suppose a new PEAR error mechanism with PEAR_Exception is used, but the PEAR project doesn't yet know how they are solving it at the time of writing. The rewritten constructor might look like this:

 function Net_Curl() {     ...     $ch = curl_init();     if (!$ch) {         throw PEAR_Exception("Couldn't initialize a new curl handle");     } } 

Besides changing the constructor, code that uses this class needs to be changed to catch the exception too, as in

 try {     $curl = new Net_Curl(); } catch {     ... } 

Unfortunately, this code will not work in PHP 4. You can support both PHP 4 and PHP 5 by using a new approach to the class implementationfor example, with a singleton pattern. An example might be

 <?php require_once "PEAR.php"; class Net_Curl {     var $type;     function Net_Curl($type) {         $this->__construct($type);     }     function __construct($type) {         $this->type = $type;     }     function singleton($type) {         if ($type == "lala") {             return PEAR::raiseError("Unable to do foo.");         } else {             return new Net_Curl($type);         }     } } $instance = Net_Curl::singleton("lala"); if (PEAR::isError($instance)) {     die("Error: " . $instance->getMessage() . "\n"); } echo $instance->type . "\n"; ?> 

Tip

To find assignments to $this in your own code, you can use the UNIX tool grep:

 egrep -r '\$this\s+=' * 

This command finds all instances in this directory and all subdirectories where an assignment to $this is made.

13.5.2. get_class

Although PHP 4 always returns the class name with lowercased letters, in PHP 5, the get_class() function returns the case-preserved version of the class name:

 <?php      class BookPage {      }      $page = new BookPage;      $name =  get_class($page);      echo $name, "\n"; ?> 

The output is bookpage in PHP 4 and BookPage in PHP 5. If you need to rely on the PHP 4 behavior, use the following code instead:

 $name = strtolower(get_class($page)); echo $name, "\n"; 

This code works for both PHP 4 and PHP 5.