Re: How to add php to a wpmu page???
There’s lots of different ways of doing this. You can create a plugin and then put your code in there. Call it from any wp page. You can also create a page template and put your code directly in the ‘search’ template. That’s probably what you are looking for.
Start here: https://codex.wordpress.org/Pages#Creating_Your_Own_Page_Templates
The general idea is that all pages are created with a specific template. You create your own, probably by copying an existing page template and modifying it, and then create a page called ‘Search’ that uses that template. You build whatever functionality you want into the template.