How do I modify existing .php files to be HTTPS compliant?

In the page that you show a url to there are links formated like -  http:// - you must change any links as Jon has pointed out to https://

<link rel="stylesheet" href="http://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css">

plus at the foot of the page:

<script src="http://cdn.jsdelivr.net/jquery.validation/1.15.0/jquery.validate.min.js"></script>

<!--Bootstrap-->

<script src="http://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/js/bootstrap.min.js"></script>

They're likely referring to the links throughout your code that still reference http vs https.

All http references should be changed to https for things like <script> links, <link> tags to your .css files or <img src=...>

If any of them are pointing to insecure content, the page could easily stop working correctly.