Comparing objects

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 19-32. 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);
?>

The above 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

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