Description:
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According to the documentation ( http://php.net/manual/en/function.is-subclass-of.php ) the third parameter of is_subclass_of decides "Whether to call autoloader if the class doesn't exist." 

That means that is_subclass_of('class1', 'class2', false) should
- return true if class1 is loaded and a subclass of class2
- return false if class1 is loaded but not a subclass of class2
- return false if class1 is not loaded

What actually happens is that it always returns false, no matter what. The naming of the parameter - "$allow_string" - suggests this is somewhat intentional, but it is a completely unintuitive and unhelpful behavior, which makes the option completely useless: is_subclass_of with $allow_string set to false becomes worthless if the first argument is a string, and if it is an object, the third parameter is pointless again as the class must have been loaded already.

Test script:
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class Super {}
class Sub extends Super {}

var_dump(is_subclass_of('Sub', 'Super')); // true

var_dump(is_subclass_of('Sub', 'Super'), false); // should be true but false