<?php

function encrypt($string, $key) {

    $strlen = strlen($string);

    $result = "";

    $flag = 0;

    for ($i = 1; $i <= $strlen; $i++) {

        $char = substr($string, $i - 1, 1);

        $keychar = substr($key, ($i % strlen($key)) - 1, 1);

        if ($flag == 0) {

            $char = chr(ord($char) + ord($keychar));

            $char = chr(ord($char) + ord($keychar));

            $flag = 1;

        } else {

            $char = chr(ord($char) + ord($keychar));

            $flag = 0;

        }

        $result .= $char;

    }

    return $result;

}

 

function decrypt($string, $key) {

    $strlen = strlen($string);

    $result = "";

    $flag = 0;

    for ($i = 1; $i <= $strlen; $i++) {

        $char = substr($string, $i - 1, 1);

        $keychar = substr($key, ($i % strlen($key)) - 1, 1);

        if ($flag == 0) {

            $char = chr(ord($char) - ord($keychar));

            $char = chr(ord($char) - ord($keychar));

            $flag = 1;

        } else {

            $char = chr(ord($char) - ord($keychar));

            $flag = 0;

        }

        $result .= $char;

    }

    return $result;

}

?>