http://natas24.natas.labs.overthewire.org
Username: natas24
Password: (natas24_password)

We examined the source page and noticed that the strcmp() function is used to verify the password.

We looked it up online to see what the strcmp() function does.
https://www.php.net/manual/en/function.strcmp.php

Next, we tried using an array instead of a string to compare and observed how the program reacted.

Then PHP displayed some warnings, and the flag appeared.

Alternatively, you can use the curl command if you prefer working from the command line.
curl -u natas24:MeuqmfJ8DDKuTr5pcvzFKSwlxedZYEWd \
-d "passwd[]" -s \
http://natas24.natas.labs.overthewire.org | grep -iE "username|password"

You got the flag! Let’s see what the next challenge has in store.