<?php

$mustget = 'Kw4SCQ==';

$key = 'frtkj';
$key_length = strlen($key);

$encoded_data = base64_decode($mustget);

$result = '';

$length = strlen($encoded_data);
for ($i = 0; $i < $length; $i++) {
    $tmp = $encoded_data[$i];
    
    for ($j = 0; $j < $key_length; $j++) {
        $tmp = chr(ord($tmp) ^ ord($key[$j]));
    }
    
    $result .= $tmp;
}

echo $result;