php encryption algorithm to achieve reversible encryption algorithm and decryption share

The encryption algorithm is as follows:

function encrypt($data, $key)
{
 $key = md5($key);
    $x  = 0;
    $len = strlen($data);
    $l  = strlen($key);
    for ($i = 0; $i < $len; $i++)
    {
        if ($x == $l) 
        {
         $x = 0;
        }
        $char .= $key{$x};
        $x++;
    }
    for ($i = 0; $i < $len; $i++)
    {
        $str .= chr(ord($data{$i}) + (ord($char{$i})) % 256);
    }
    return base64_encode($str);
}

The decryption algorithm is as follows:

function decrypt($data, $key)
{
 $key = md5($key);
    $x = 0;
    $data = base64_decode($data);
    $len = strlen($data);
    $l = strlen($key);
    for ($i = 0; $i < $len; $i++)
    {
        if ($x == $l) 
        {
         $x = 0;
        }
        $char .= substr($key, $x, 1);
        $x++;
    }
    for ($i = 0; $i < $len; $i++)
    {
        if (ord(substr($data, $i, 1)) < ord(substr($char, $i, 1)))
        {
            $str .= chr((ord(substr($data, $i, 1)) + 256) - ord(substr($char, $i, 1)));
        }
        else
        {
            $str .= chr(ord(substr($data, $i, 1)) - ord(substr($char, $i, 1)));
        }
    }
    return $str;
}

The above encryption and decryption process requires 1 encryption key (that is, the parameter $key).

$data = 'PHP Encryption and decryption algorithm ';  //  Encrypted message 
$key = '123';     //  The key 
$encrypt = encrypt($data, $key);
$decrypt = decrypt($encrypt, $key);
echo $encrypt, "\n", $decrypt;

The above output will be similar to the following results:

gniCSOzZG+HnS9zcFea7SefNGhXF
PHP Encryption and decryption algorithm 

It can be seen from the above results that this is a set of reversible encryption and decryption algorithms, which can be used for partial data encryption that needs to be restored.