Do the robots see the PHP code ?
The search engines bots browse the pages of your website. You server sends the content to bots when they access the website.
The content sent from your server is never PHP code but the code generated by PHP (or by another similar language), in other words, it is HTML.
To make it simpler, bots visit your website just like any other visitor. If they had access to the source code of each website, that would be a huge security hole.
So there is no need to name variables with keywords :-).