${@var_dump(md5(120902694))}; -

If this were executed on a vulnerable server, the output would look like this: string(32) "f91289c99fe56ec5f183dfefe39ecda8" Why do people use this?

If the string f91289c99fe56ec5f183dfefe39ecda8 appears on the page after posting, it proves the site is insecure and could be fully compromised by an attacker. ${@var_dump(md5(120902694))};

While this specific string is a common "signature" for scanners, it's generally harmless on its own unless the server is misconfigured to run it. If this were executed on a vulnerable server,

Specifically, the command var_dump(md5(120902694)) tells the server to calculate a unique fingerprint (an MD5 hash ) for that number and display the result along with its data type. ${@var_dump(md5(120902694))};