This has already been mentioned (see jazfresh at hotmail.com's note), but here it is again in more detail because for objects the difference between == and === is significant.
Loose equality (==) over objects is recursive: if the properties of the two objects being compared are themselves objects, then those properties will also be compared using ==.
<?php
class Link
{
public $link; function __construct($link) { $this->link = $link; }
}
class Leaf
{
public $leaf; function __construct($leaf) { $this->leaf = $leaf; }
}
$leaf1 = new Leaf(42);
$leaf2 = new Leaf(42);
$link1 = new Link($leaf1);
$link2 = new Link($leaf2);
echo "Comparing Leaf object equivalence: is \$leaf1==\$leaf2? ", ($leaf1 == $leaf2 ? "Yes" : "No"), "\n";
echo "Comparing Leaf object identity: is \$leaf1===\$leaf2? ", ($leaf1 === $leaf2 ? "Yes" : "No"), "\n";
echo "\n";
echo "Comparing Link object equivalence: is \$link1==\$link2? ",($link1 == $link2 ? "Yes" : "No"), "\n";
echo "Comparing Link object identity: is \$link1===\$link2? ", ($link1 === $link2 ? "Yes" : "No"), "\n";
?>
Even though $link1 and $link2 contain different Leaf objects, they are still equivalent because the Leaf objects are themselves equivalent.
The practical upshot is that using "==" when "===" would be more appropriate can result in a severe performance penalty, especially if the objects are large and/or complex. In fact, if there are any circular relationships involved between the objects or (recursively) any of their properties, then a fatal error can result because of the implied infinite loop.
<?php
class Foo { public $foo; }
$t = new Foo; $t->foo = $t;
$g = new Foo; $g->foo = $g;
echo "Strict identity: ", ($t===$g ? "True" : "False"),"\n";
echo "Loose equivalence: ", ($t==$g ? "True" : "False"), "\n";
?>
So preference should be given to comparing objects with "===" rather than "=="; if two distinct objects are to be compared for equivalence, try to do so by examining suitable individual properties. (Maybe PHP could get a magic "__equals" method that gets used to evaluate "=="? :) )
