You could accomplish this by setting up a redirect. If you’re using an Apache server, for your example, you would add this to .htaccess above the WordPress block:
RewriteEngine On
RewriteRule somepost.php ^/somepost/
It’s much more common, and recommended, to create a PHP file and a Page (or other post type) within wp-admin). This will still run all of your PHP code, and has the added benefit of automatically including all the WordPress functionality as opposed to an orphan PHP file. In somepost.php in your theme folder:
<?php
/* Template Name: Show Some Post
*/
// all your PHP here
?>
then within wp-admin, actually create a Page (or another post type that supports page-attributes) and select this particular Page Template. You will then have a URL such as http://examplepage.com/somepost/.