3.5、unicode转utf8

/**
 * unicode转utf8
 * @param  $c
 */
function unicode_to_utf8($c) {
   $str = '';
   if($c < 0x80) {
      $str .= $c;
   } elseif($c < 0x800) {
      $str .= (0xC0 | $c >> 6);
      $str .= (0x80 | $c & 0x3F);
   } elseif($c < 0x10000) {
      $str .= (0xE0 | $c >> 12);
      $str .= (0x80 | $c >> 6 & 0x3F);
      $str .= (0x80 | $c & 0x3F);
   } elseif($c < 0x200000) {
      $str .= (0xF0 | $c >> 18);
      $str .= (0x80 | $c >> 12 & 0x3F);
      $str .= (0x80 | $c >> 6 & 0x3F);
      $str .= (0x80 | $c & 0x3F);
   }
   return $str;
}