$sql_set = [];
if($_POST['pol']) $sql_set [] = ' `pol` = "momche" ';
if($_POST['age']) $sql_set [] = ' `age` = "56" ';
if($_POST['names']) $sql_set [] = ' `names` = "voTkaPoweR" ';
mysqli_query($db, 'UPDATE users SET '.implode(', ', $sql_set).' WHERE id = '.$user_id.'');