Comparing objects in PHP 5

In PHP 5, object comparison is a 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, defined in the same namespace.

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 (in a particular namespace).

An example will clarify these rules.

Example 13-3. 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 { var $flag; function Flag($flag=true) { $this->flag = $flag; } } namespace Other { class Flag { var $flag; function Flag($flag=true) { $this->flag = $flag; } } } $o = new Flag(); $p = new Flag(); $q = $o; $r = new Other::Flag(); echo "Two instances of the same class\n"; compareObjects($o, $p); echo "\nTwo references to the same instance\n"; compareObjects($o, $q); echo "\nInstances of similarly named classes in different namespaces\n"; compareObjects($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 similarly named classes in different namespaces o1 == o2 : FALSE o1 != o2 : TRUE o1 === o2 : FALSE o1 !== o2 : TRUE