Classes implementing Countable can be used with the count() function.
I just want to point out that your class has to actually implement the Countable interface, not just define a count method, to be able to use count($object) and get the expected results. I.e. the first example below won't work as expected, the second will. (The normal arrow function accessor ($object->count()) will work fine, but that's not the kewl part :) )
<?php
//Example One, BAD :(
class CountMe
{
    protected $_myCount = 3;
    public function count()
    {
        return $this->_myCount;
    }
}
$countable = new CountMe();
echo count($countable); //result is "1", not as expected
//Example Two, GOOD :)
class CountMe implements Countable
{
    protected $_myCount = 3;
    public function count()
    {
        return $this->_myCount;
    }
}
$countable = new CountMe();
echo count($countable); //result is "3" as expected
?>