php overload equals-operator

Question

In a PHP program I have an array of some custom objects, and I want to find if the array contains a certain object. Of course I can use array_search, but this checks if the objects are the same object, not if it has the same variables. So I want to be able to create my own compare function for the objects, which I can use with the array_search method (or something similar). I want to be able to do something like this:

class foo
{
    public $_a,$_b;
    function __construct($a,$b)
    {
        $this->_a = $a;
        $this->_b = $b;
    }

    function __equals($object)
    {
        return $this->_a == $object->_a;
    }
}
$f1 = new foo(5,4);
$f2 = new foo(4,6);
$f3 = new foo(4,5);

$array = array($f1,$f2);
$idx = array_search($f3,$array); // return 0

Is something like this possible? I know I can also create my own array_search method which uses a method from the class, but than I'd have to use 2 different search functions, one for the classes which do have their own compare function, and one for those which haven't.