You don’t. Php is a server-side scripting language. It is executed (parsed, tokenized, and interpreted) on the server when the page is requested. The purpose of php is to build and output (dynamic) web pages and perform actions on the server.
The css and javascrpt link and script tags are html markup. The target urls in them are requested by the browser, where they are then parsed and rendered/executed in the browser.
You can use a url to a php page in the html markup, but when the browser requests that url, the php code is executed on the server and only the output from that php code is sent to the browser.
If this and the other replies do not address your question, you will need to be more specific about what it is you are trying to accomplish or what problem you are trying to solve.
That probably explains things well enough. I will add to that in case it helps.
For a HTML file the server reads the file and sends it to the client as-is. The browser gets the file the same or nearly the same as it exists in the server.
Many languages, including PHP, C# and VB.Net, are executed server-side, as mabismad says. For them the server reads the file but during the reading it compiles and executes the relevant code when it exists in the file.