Creating custom page template from existing PHP site

It really is rather simple. Check out the Codex on Page Templates for more info.

1. Make a copy the existing page.php file located inside your WordPress theme’s folder.
2. Rename your page.php file to page-mypage.php
3. At the very top of your new page-mypage.php file, right after the opening <?php tag add the following code to define your custom page template.

/* Template Name: My Page Template */

4. Now inside your page-mypage.php file locate the WordPress loop. It usually looks like this:

   <?php
   // Start the loop.
   while ( have_posts() ) : the_post();
   
       // Include the page content template.
       get_template_part( 'template-parts/content', 'page' );
   
       // If comments are open or we have at least one comment, load up the comment template.
       if ( comments_open() || get_comments_number() ) {
           comments_template();
       }
   
       // End of the loop.
   endwhile;
   ?>
   

5. Replace the entire loop with the relevant PHP code of your project

6. Now in WordPress you will have to add a new page named mypage and select your custom template (My Page Template) in the “Page Attributs” sidebar block. There will be a dropdown menu called “Template”.
7. Done!