Something

function clean($string) {
  $badChars = array("'","\"","<",">",".",",","!","@",
        "#","$","%","^","&","*","(",")","_",
        "-","=","+");

  $length = strlen($string);

  for($i=0;$i<=$length;$i++) {
    $curChar = substr($string,$i,1);

    for($j=0;$j<=$length;$j++) {
      if($badChars[$j] == $curChar) {
        die("<b>string is bad!</b> <font size=2>(error at char: $j)</font>");
      }
    }
  }

  return $string;
}