URL and location within directory structure for a custom page.

I'm planning to write a custom page, which will resemble items/browse.php but which will display more information than the title for each item and will contain a loop within a loop.

I can't use the Simple Pages plugin to create this page because it will contain php.

Where should I put this page in Omeka's directory structure and how can I get Omeka to recognise the url? For example Omeka won't open [mydomain]/my-custom-page.php. Does it need to be part of a plugin?

You can use Simple Pages for it if you create a Shortcode for the PHP, taking any necessary input as arguments. Check out the docs for working with shortcodes.

Depending on what you're looking to accomplish, you might also be able to use conditionals in items/browse.php. I've done that to show collection info above the results and to display the results differently for certain collections.