function_exists() does not cache its query internally.

by executing the following code

        $funcs = \array_shift(\get_defined_functions());
        $l = new \core\utiles\loadTime;
        $times = 0;
        $l->start();
        for($index = 0; $index<count($funcs); $index++)
        {
            foreach($funcs as $func)
            {
                $times++;
                if(\function_exists($func)) ;
            }
        }
        $s = $l->stop();
        echo "<span style='color:green'>$times</span> took : $s";
        # the output would be 
         $> 2365444 took : 0.70324

By executing the 

        $funcs = \array_shift(\get_defined_functions());
        $l = new \core\utiles\loadTime;
        $times = 0;
        $l->start();
        static $func_check = array();
        for($index = 0; $index<count($funcs); $index++)
        {
            foreach($funcs as $func)
            {
                $times++;
                if(!isset($func_check[$func]))
                {
                    if(\function_exists($func)) ;
                    $func_check[$func] = 1;
                }
                else $func_check[$func];
            }
        }
        $s = $l->stop();
        echo "<span style='color:green'>$times</span> took : $s";
        # the output would be 
        $> 2365444 took : 0.53446

There is a 0.16878 seconds improvement, when you use static array to cache methods existence.