How to add a PHP page to WordPress?

Solution for : I want to create a custom page for my WordPress blog that will execute my PHP code in it, whilst remaining a part of the overall site CSS/theme/design.

First, duplicate post.php or page.php in your theme folder (under /wp-content/themes/themename/).

Rename the new file as templatename.php (where templatename is what you want to call your new template). To add your new template to the list of available templates, enter the following at the top of the new file:

/*
Template Name: Name of Template
*/

You can modify this file (using PHP) to include other files or whatever you need.

Then create a new page in your WordPress blog, and in the page editing screen you'll see a Template drop-down in the Attributes widget to the right. Select your new template and publish the page.

Your new page will use the PHP code defined in templatename.php