function replace_keep_case($st1, $find1) {

/// set prefix and suffix here
$prefix="";
$suffix="";

$index=0;
$resultstring="";
   while ($index < strlen($st1)) {
      if (strtoupper(substr($st1, $index, strlen($find1)))==strtoupper($find1)) {
        $resultstring.=$prefix.substr($st1, $index, strlen($find1)). $suffix;
        $index=$index+strlen($find1); 
      } else {
        $resultstring.=substr($st1, $index, 1);
        $index++;
      }
    }
   return $resultstring; 
}