Web spiders can spider PHP pages.
But they won't go infinitely deep into your site (obviously), especially if you use query strings, especially if you have many similar pages.
Tips:
1. Make each page have JUST ONE URL. So always be totally consistent when you refer to it. So if you have blog.php?id=1 don't also have blog.php?id=1&useless=42 or other URLs pointing to the same item. Also watch for stuff like blog.php?cat=1&id=2 and blog.php?id=2&cat=1 which are DIFFERENT URLs.
Search engines have limited resources to spider your site - ensure that they use them wisely.
2. Use robots.txt to ban robots from pages that are of no interest to them. This is particularly relevant if you have something like logon.php?from=somepage.php linked from somepage.php - You don't want to double the number of pages on your site with links to logon.php with different query strings.
Normal pages to ban robo
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