Имя функции из переменной в пхп

$function_name = "$a_$b";
$result = 0;
if(function_exists($function_name))$result = $function_name();
$function_name = "$b_$a";
if(function_exists($function_name) && !$result)$result = $function_name();

$function_name = "$a_$b";