$_GET and URL Rewriting for PHP

Question

How does URL rewriting affect the $_GET parameter for PHP?

Say I have a URL like http://example.com/index.php?p=contact and I use $_G

Answer 1

I'd amend Grant's answer to "Yes, that will work mostly as expected."

Specifically, mod_rewrite's behavior with regards to existing query strings can be surprising. As an example, let's take the following rule, which converts the URL you supplied:

RewriteRule /contact /index.php?p=contact

This will correctly rewrite /contact to /index.php?p=contact and the page name will be accessible through $_GET['p']. However, if you use this technique with a script that uses parameters other than the page name, it gets slightly trickier. This rule also translates /contact?person=Joe to /index.php?p=contact. The person=Joe parameter disappears entirely! There are two ways to deal with this.

The simplest way is to use the [QSA] ("query string append") flag on your rule, which will place the original query string after the parameters supplied in the rule, translating /contact?person=Joe to /index.php?p=contact&person=Joe:

RewriteRule /contact /index.php?p=contact [QSA]

However, this makes it possible for your p= parameter to get overwritten. Visiting /contact?p=about will get rewritten to /index.php?p=contact&p=about, so $_GET['p'] will return "about" in your script, not "contact". To resolve this, use the QUERY_STRING variable instead:

RewriteRule /contact /index.php?%{QUERY_STRING}&p=contact

This guarantees that $_GET['p'] will always return "contact" when using this rule, regardless of whether your visitors are messing with your URLs. :-)