$arrEn=Array("A","B","C","D","a","b","c","d","и т.д.");
$arrRus=Array("А","Б","В","Г","а","б","в","г","и т.д.");
$en=false;$ru=false;
for($i=0;$i<strlen($str);$i++)
{
if(in_array($str[$i],$arrEn)
$en=true;
if(in_array($str[$i],$arrRus)
$ru=true;
if($en && $ru)
die("ОШИБКО!!!!!");
}