static public function elapsedTime($start, $end) {
        $start = isset(self::$_marks[$start]) ? self::$_marks[$start]['time'] : 0;
        $end = isset(self::$_marks[$end]) ? self::$_marks[$end]['time'] : 0;

        return $start ? round($end - $start, 10) : null;
    }
static public function elapsedTime($start, $end) {
        $start = isset(self::$_marks[$start]) ? self::$_marks[$start]['time'] : 0;
        $end = isset(self::$_marks[$end]) ? self::$_marks[$end]['time'] : 0;

        return $start ? round($end - $start, 10) : null;
    }
static public function elapsedTime ( $ start , $ end ) { $ start = isset ( self :: $ _marks [ $ start ] ) ? self :: $ _marks [ $ start ] [ 'time' ] : 0 ; $ end = isset ( self :: $ _marks [ $ end ] ) ? self :: $ _marks [ $ end ] [ 'time' ] : 0 ; return $ start ? round ( $ end - $ start , 10 ) : null ; }
Compute the elapsed time of two marks

@param   string  $start  Keyname 1
@param   string  $end  Keyname 2

@return  mixed
Compute the elapsed time of two marks