Ето ти нещо което се надявам да помогне :
<?php
include "functions.php";
capital();
$latin = array('ч','щ','ш','ю','а','б','в','г','д','е','ж','з','и','й','к'
,'л','м','н','о','п','р','с','т','у','ф','х','ц','ъ','ь','я',
'в','Ч','Щ','Ш','Ю','А','Б','В','Г','Д','Е','Ж','З','И','Й',
'К','Л','М','Н','О','П','Р','С','Т','У','Ф','Х','Ц','Ъ','Ь','Я','В','ч','ш','щ');
$cyrilic = array('ch','sht','sh','iu','a','b','v','g','d','e','j','z','i','i',
'k','l','m','n','o','p','r','s','t','u','f','h','c','y','y',
'q','w','Ch','Sht','Sh','Iu','A','B','V','G','D','E','J','Z',
'I','I','K','L','M','N','O','P','R','S','T','U','F','H','C','Y','Y','Q','W','4','6','6t');
?>
$search = htmlspecialchars($_POST['search']);
$retext = addslashes(str_replace($cyrilic, $latin, $search));