<?php
$strings1 = 'Арг.ен!тина ма,нит нег$ра';
$strings2 = 'S!um summ$us mu,s';
$strings3 = 'Арг.ен!тина ма,нит нег$ра'.'S!um summ$us mu,s';
$strings1 = mb_strtolower(preg_replace('/[^\p{Cyrillic}\p{Latin}]/u', '', $strings1));
var_dump($strings1);
$strings2 = mb_strtolower(preg_replace('/[^\p{Cyrillic}\p{Latin}]/u', '', $strings2));
var_dump($strings2);
$strings3 = mb_strtolower(preg_replace('/[^\p{Cyrillic}\p{Latin}]/u', '', $strings3));
var_dump($strings3);
preferences:
68.98 ms | 402 KiB | 5 Q