Similarly to lfi-ctf-1, we're dealing with the PHP application that includes the file. In this case the file is provided as a GET query param.
I've tried several filenames to see if there's something worthy looking, only to get to know that everything that is not the letter is escaped (numbers, special characters and dots are filtered out of the string). so the GET parameter was a no go.
However, the hint suggested that the website uses PHPs' $_REQUEST to accept HTTP requests. I am not a PHP developer, but from the docs I got to know that $_REQUEST had contents of both $_GET and $_POST (and $_COOKIE, but that's doesn't matter here).
I've ran Burp Suite to intercept the GET request sent from the website form and edited it to be POST one with welcome2 as a string. Fortunately, with POST, the PHP code tried to include the file without filtering out the number (2).
So I've tried to include /etc/flag3%00 (flag file with null-byte) with success: