PHP实现ASCII码与字符串相互转换的方法

<?php
class ascii {
  /
    将ascii码转为字符串
    @param type $str 要解码的字符串
    @param type $prefix 前缀，默认:&#
    @return type
   /
  function decode($str, $prefix="&#") {
    $str = str_replace($prefix, "", $str);
    $a = explode(";", $str);
    foreach ($a as $dec) {
      if ($dec < 128) {
        $utf .= chr($dec);
      } else if ($dec < 2048) {
        $utf .= chr(192 + (($dec - ($dec % 64)) / 64));
        $utf .= chr(128 + ($dec % 64));
      } else {
        $utf .= chr(224 + (($dec - ($dec % 4096)) / 4096));
        $utf .= chr(128 + ((($dec % 4096) - ($dec % 64)) / 64));
        $utf .= chr(128 + ($dec % 64));
      }
    }
    return $utf;
  }
  /
    将字符串转换为ascii码
    @param type $c 要编码的字符串
    @param type $prefix 前缀，默认&#
    @return string
   /
  function encode($c, $prefix="&#") {
    $len = strlen($c);
    $a = 0;
    while ($a < $len) {
      $ud = 0;
      if (ord($c{$a}) >= 0 && ord($c{$a}) <= 127) {
        $ud = ord($c{$a});
        $a += 1;
      } else if (ord($c{$a}) >= 192 && ord($c{$a}) <= 223) {
        $ud = (ord($c{$a}) - 192)  64 + (ord($c{$a + 1}) - 128);
        $a += 2;
      } else if (ord($c{$a}) >= 224 && ord($c{$a}) <= 239) {
        $ud = (ord($c{$a}) - 224)  4096 + (ord($c{$a + 1}) - 128)  64 + (ord($c{$a + 2}) - 128);
        $a += 3;
      } else if (ord($c{$a}) >= 240 && ord($c{$a}) <= 247) {
        $ud = (ord($c{$a}) - 240)  262144 + (ord($c{$a + 1}) - 128)  4096 + (ord($c{$a + 2}) - 128)  64 + (ord($c{$a + 3}) - 128);
        $a += 4;
      } else if (ord($c{$a}) >= 248 && ord($c{$a}) <= 251) {
        $ud = (ord($c{$a}) - 248)  16777216 + (ord($c{$a + 1}) - 128)  262144 + (ord($c{$a + 2}) - 128)  4096 + (ord($c{$a + 3}) - 128)  64 + (ord($c{$a + 4}) - 128);
        $a += 5;
      } else if (ord($c{$a}) >= 252 && ord($c{$a}) <= 253) {
        $ud = (ord($c{$a}) - 252)  1073741824 + (ord($c{$a + 1}) - 128)  16777216 + (ord($c{$a + 2}) - 128)  262144 + (ord($c{$a + 3}) - 128)  4096 + (ord($c{$a + 4}) - 128)  64 + (ord($c{$a + 5}) - 128);
        $a += 6;
      } else if (ord($c{$a}) >= 254 && ord($c{$a}) <= 255) { //error
        $ud = false;
      }
      $scill .= $prefix.$ud.";";
    }
    return $scill;
  }
}
/
 PHP 转 ASCII
 require_once "ascii_class.php";
/
$aa = new ascii;
echo "<xmp>";
echo $str = $aa->encode("PHP编程.jb51.");
echo "</xmp>";
echo $aa->decode($str);
?>