I tried adding a PHP script inside BSS with the Custom Code from the Misc menu on the left. I simply did
<?php echo "test"; ?> where it should've said test on the page.
I see the default site format is html (bottom right), is there a way to change them inside BSS? @Jo recommended me to use custom code blocks, I was gonna give that a try. But if no one has a solution to this I might move on to Sublime. I just need the pages in .php instead.