<?
function is_utf8($string) {
   // From http://w3.org/International/questions/qa-forms-utf-8.html
   return preg_match('%^(?:
         [\x09\x0A\x0D\x20-\x7E]            # ASCII
       | [\xC2-\xDF][\x80-\xBF]            # non-overlong 2-byte
       |  \xE0[\xA0-\xBF][\x80-\xBF]        # excluding overlongs
       | [\xE1-\xEC\xEE\xEF][\x80-\xBF]  # straight 3-byte
       |  \xED[\x80-\x9F][\x80-\xBF]        # excluding surrogates
       |  \xF0[\x90-\xBF][\x80-\xBF]    # planes 1-3
       | [\xF1-\xF3][\x80-\xBF]          # planes 4-15
       |  \xF4[\x80-\x8F][\x80-\xBF]    # plane 16
   )*$%xs', $string);  
}?>