PHC:Polymorphic Encryption

<?php
class PHC
{
    # encrypt some text
    public function encrypt( $plain ) {
        $i = 8;
        $key = '';
        $chars = array(range('A','Z'),range('a','Z'));
        while($i > 0) {
            $key = array_rand($chars[rand(1,0)]);
            $i--;
        }
        $block = '';
        $i = 0;
        $max = strlen($key) - 1;
        foreach(str_split($plain) as $index => $char) {
            if(rand(0,20) < 19) {
                $block .= "\\x".($index)."\\x".$char.$key[$i];
            } else {
                $block .= "\\x".($index)."\\x".$char.$key[$i].$block;
            }
            $i = ($i + 1 > $max)?0:$i+1;
        }
        return $block;
    }

    # decrypt some text
    public function decrypt( $encrypted ) {
        #$encrypted[strlen($encrypted)-1] = '';
        $enc = $encrypted;
        $blocks = array();
        while(preg_match("#(\\\\x[0-9]+\\\\x(.|\.)*?)\\\\x#", $encrypted, $chunk)) {
            $c = str_replace("\\x", "\\\\x", $chunk[1]);
            $encrypted = preg_replace('#'.str_replace("\\x", "\\\\x", $chunk[1]).'#', '', $encrypted, 1);
            $temp = explode("\\x", $chunk[1]);
            $tmp = str_split($temp[2]);
            $blocks[(int)$temp[1]] = $tmp[0];
        }
        $array = explode("\\x", $enc);
        $last = array(array_pop($array), array_pop($array));
        $blocks[$last[1]] = $last[0];
        ksort($blocks);
        return implode('', $blocks);
    }
}
$phc = new PHC;
$var = $phc->encrypt('Hello world');
echo "{$var}<hr />";
echo $phc->decrypt($var);
?>