Comparing objects

In PHP 5, object comparison is more complicated than in PHP 4 and more in accordance to what one will expect from an Object Oriented Language (not that PHP 5 is such a language).

When using the comparison operator (==), object variables are compared in a simple manner, namely: Two object instances are equal if they have the same attributes and values, and are instances of the same class.

On the other hand, when using the identity operator (===), object variables are identical if and only if they refer to the same instance of the same class.

An example will clarify these rules.

Example 19-33. Example of object comparison in PHP 5 <?php function bool2str($bool) {     if ($bool === false) {         return 'FALSE';     } else {         return 'TRUE';     } } function compareObjects(&$o1, &$o2) {     echo 'o1 == o2 : ' . bool2str($o1 == $o2) . "\n";     echo 'o1 != o2 : ' . bool2str($o1 != $o2) . "\n";     echo 'o1 === o2 : ' . bool2str($o1 === $o2) . "\n";     echo 'o1 !== o2 : ' . bool2str($o1 !== $o2) . "\n"; } class Flag {     public $flag;     function Flag($flag = true) {         $this->flag = $flag;     } } class OtherFlag {     public $flag;     function OtherFlag($flag = true) {         $this->flag = $flag;     } } $o = new Flag(); $p = new Flag(); $q = $o; $r = new OtherFlag(); echo "Two instances of the same class\n"; compareObjects($o, $p); echo "\nTwo references to the same instance\n"; compareObjects($o, $q); echo "\nInstances of two different classes\n"; compareObjects($o, $r); ?>

This example will output:

Two instances of the same class o1 == o2 : TRUE o1 != o2 : FALSE o1 === o2 : FALSE o1 !== o2 : TRUE Two references to the same instance o1 == o2 : TRUE o1 != o2 : FALSE o1 === o2 : TRUE o1 !== o2 : FALSE Instances of two different classes o1 == o2 : FALSE o1 != o2 : TRUE o1 === o2 : FALSE o1 !== o2 : TRUE