PHP ASCII码与字符串的相互转换

<?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二次开发：www.php2.cc");

echo "</xmp>";
echo $aa->decode($str);
?>