@Jonathan3
I have actually created two PHP modules, one is a contact form and one is a the "call now".
I can access the contact form by example.com/contact_form/contact_form.php
but in contrast, example.com/call_now/call_now.php
is of course something which is not a standalone webpage, rather just a caller for some HTML-CSS.
I can understand why you recommend to edit a skin's main PHP file; in theory I could paste all the PHP code currently in call_now.php
inside the skin's main PHP file but the problem with that is that it each time I would upgrade MediaWiki I would have to re-paste it there again.
I think that what I need is to to execute my code from within MediaWiki GUI somehow (just as I paste JavaScript in Common.js and CSS in Common.css).