php-doc-en/language/oop5/object-comparison.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- $Revision$ -->
  <sect1 xml:id="language.oop5.object-comparison" xmlns="http://docbook.org/ns/docbook">
   <title>Comparing Objects</title>
   <para>
    When using the comparison operator (<literal>==</literal>), 
    object variables are compared in a simple manner, namely: Two object
    instances are equal if they have the same attributes and values (values are compared with <literal>==</literal>), and are
    instances of the same class.
   </para>
   <para>
    When using the identity operator (<literal>===</literal>),
    object variables are identical if and only if they refer to the same
    instance of the same class.
   </para>
   <para>
    An example will clarify these rules.
    <example>
     <title>Example of object comparison</title>
     <programlisting role="php">
<![CDATA[
<?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 __construct($flag = true) {
        $this->flag = $flag;
    }
}

class OtherFlag
{
    public $flag;

    function __construct($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);
?>
]]>
     </programlisting>
     &example.outputs;
     <screen>
<![CDATA[
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
]]>
     </screen>
    </example>
   </para>
   <note>
    <para>
     Extensions can define own rules for their objects comparison
     (<literal>==</literal>).
    </para>
   </note>
  </sect1>
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