ctf.txt

The PHP code is looking for a query parameter with the name "get_param" and the value "Hello world!".

I believe you can do this by appending the following to the URL of this PHP script: `?get_param=Hello world!`.
If that doesn't work you may need to URL-encode the "Hello world!" (because normally spaces aren't allowed in query params — but I think if you type
this into your browser location bar it will do the conversion for you automatically).
The URL-encoded form can be found by using the JavaScript `encodeURIComponent` function, like so: `encodeURIComponent('Hello world!')`.
The value is "Hello%20world!".

Good luck!