$bul="Türkçe Metin";
$bulunacak = array('ç','Ç','ı','İ','ğ','Ğ','ü','ö','Ş','ş','Ö','Ü',',',' ','(',')','[',']');
$degistir = array('c','C','i','I','g','G','u','o','S','s','O','U','','-','','','','');
$sonuc=str_replace($bulunacak, $degistir, $bul);
echo $sonuc;