In `Hack.lu 2018 - BabyPHP` challenge, there is an `unsanitized user input` vulnerability which results in `unintended behaviors` as well as `code injection`. First, we can provide a `data:` URL to `file_get_contents` to return the required value. Then, we should pass `Array` in the parameter, so we force `substr` and `sha1` return `null`. Also, we can override the values of arbitrary variables using `$$` in `PHP`. Finally, we can run arbitrary code by passing arbitrary `$bb` value into `assert` in order to print `$flag`. This is an interesting `web` challenge to learn how to attack `PHP` applications.