Comparing objects in PHP 5

Warning

This extension is EXPERIMENTAL. The behaviour of this extension -- including the names of its functions and anything else documented about this extension -- may change without notice in a future release of PHP. Use this extension at your own risk.

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.

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 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;
    }
}

class OtherFlag {
    var $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