How to mathematically write "a sequence of adjacent elements (numbers)"?

Question

PHP

For example, there is A = {a1, a2, a3, ... , an}
And you need to find B = {b1, b2, ... , bn} in A, but such that the sequence of adjacent elements (numbers)

Answer 1

Post the full line. In general,
\x specifies hexadecimal notation.
Look at the ASCII table to see what 0x00 and similar characters represent.
You can try this:
(c) Original

foreach (str_split(substr($string, [start of data]), 2) as $chunk) {
    $d = unpack("H*data", $chunk);
    $array[] = hexdec($d["data"]);
}

Answer 2

yes, first you change eval to echo. this is the first level. in s-ti from what will give out - then you have to look again.
deciphered many such backdoors, there is little point in this. in the end, there will be code that, through eval or somehow else, executes code from a cookie with a certain name, or from a post parameter.
this is just a backdoor - it only gives a hole, and what will be done through it - can only be understood by decoding the code, understanding where the code will come from and putting a trap instead that will save this data. It remains only to wait for the attacker to come to use it. But if you do not execute the sent code and do not give a response, the attacker will make a simple request, for example, a php version, will see that it does not work and leave. the code needs to be executed and returned, then it will be possible to see what it does there.