实现PHP 转 ASCII

<?php class ascii{function decode($str){    preg_match_all( "/(d{2,5})/", $str,$a);    $a = $a[0];     foreach ($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 $utf;}function encode($c){    $len = strlen($c);    $a = 0;     while ($a < $len)     {        $ud = 0;         if (ord($c{$a}) >=0 && ord($c{$a})=192 && ord($c{$a})=224 && ord($c{$a})=240 && ord($c{$a})=248 && ord($c{$a})=252 && ord($c{$a})=254 && ord($c{$a}) encode("a");?>