[PHP] add is_a() 3rdargument for string types

[TomasVotruba]

 $parentClassName = 'SomeClass;
-var_dump(is_a($parentClassName, 'SomeClass')); // false
+var_dump(is_a($parentClassName, 'SomeClass', true)); // true

The first check is actually false positive, because we desire it to check the string.