Description:
------------
When implicitely casting class_exists() to a boolean, a memory leak occurs. It does not matter if the given class exists or not.

Reproduce code:
---------------
for ($i = 1; $i < 100; $i++) {

    if (class_exists('foo')) { }
    echo memory_get_usage() . "\n";	
	
}


Expected result:
----------------
A constant amount of memory (at least after the first iteratation)

Actual result:
--------------
A growing amount of used memory.

A workaround can be:

if ((boolean)class_exists('foo')) { }

or

$foo = class_exists('foo');
if ($foo) {} 

in these cases the memory leak does not occur