$string = "äáčé";
$convert = Array(
'ä'=>'a',
'Ä'=>'A',
'á'=>'a',
'Á'=>'A',
'à'=>'a',
'À'=>'A',
'ã'=>'a',
'Ã'=>'A',
'â'=>'a',
'Â'=>'A',
'č'=>'c',
'Č'=>'C',
'ć'=>'c',
'Ć'=>'C',
'ď'=>'d',
'Ď'=>'D',
'ě'=>'e',
'Ě'=>'E',
'é'=>'e',
'É'=>'E',
'ë'=>'e',
);
$string = strtr($string , $convert );
echo $string; //aace