Notice:  Uninitialized string offset: 63 in [...][...] on line 18
Notice:  Uninitialized string offset: 63 in [...][...] on line 19

$tmp = md5(md5($key.$strofsym[$x-1]).$key);
$encoded = str_replace($tmp[3].$tmp[6].$tmp[1].$tmp[2], $strofsym[$x-1], $encoded);

function encode($unencoded,$key){ //Шифрование
$string=base64_encode($unencoded);
$newstr = '';
$arr=array();
$x=0;
while ($x++< strlen($string)) {
$arr[$x-1] = md5(md5($key.$string[$x-1]).$key);
$newstr = $newstr.$arr[$x-1][3].$arr[$x-1][6].$arr[$x-1][1].$arr[$x-1][2];
}
return $newstr;
}

function decode($encoded, $key){ //Расшифровка
$strofsym='qwertyuiopasdfghjklzxcvbnm1234567890QWERTYUIOPASDFGHJKLZXCVBNM=';
$x=0;
while ($x++<= strlen($strofsym)) {
$tmp = md5(md5($key.$strofsym[$x-1]).$key);
$encoded = str_replace($tmp[3].$tmp[6].$tmp[1].$tmp[2], $strofsym[$x-1], $encoded);
}
return base64_decode($encoded);
}

echo decode('70072bbbd45b597a700751f437683768','z');