Apache Rewrite Rule

RewriteEngine on
RewriteRule ^page/([^/\.]+)/?$ index.php?page=$1 [L]

Let’s walk through that RewriteRule, and work out exactly what’s going on:

^page/

Sees whether the requested page starts with page/. If it doesn’t, this rule will be ignored.

([^/.]+)

Here, the enclosing brackets signify that anything that is matched will be remembered by the RewriteRule. Inside the brackets, it says “I’d like one or more characters that aren’t a forward slash or a period, please”. Whatever is found here will be captured and remembered.

/?$

Makes sure that the only thing that is found after what was just matched is a possible forward slash, and nothing else. If anything else is found, then this RewriteRule will be ignored.

index.php?page=$1

The actual page which will be loaded by Apache. $1 is magically replaced with the text which was captured previously.

[L]

Tells Apache to not process any more RewriteRules if this one was successful.

Let’s write a quick page to test that this is working. The following test script will simply echo the name of the page you asked for to the screen, so that you can check that the RewriteRule is working.

Second mod_rewrite example
         The requested page was:   

Again, upload both the index.php page, and the .htaccess file to the same directory. Then, test it! If you put the page in http://www.somesite.com/mime_test/, then try requesting http://www.somesite.com/mime_test/page/software. The URL in your browser window will show the name of the page which you requested, but the content of the page will be created by the index.php script! This technique can obviously be extended to pass multiple query strings to a page - all you’re limited by is your imagination.

Conditional Statements and mod_rewrite

But what happens when you start getting people hotlinking to your images (or other files)? Hot linking is the act of including an image, media file, etc from someone else’s server in one of your own pages as if it were your own. Obviously, as a webmaster, there are plenty of times when you don’t want people doing that. You’ll almost certainly have seen examples where someone has linked to one image on a website, only for a completely different, “nasty” one to be shown instead. So, how is this done?

It’s pretty simple really. All it takes are a couple of RewriteCond statements in your .htaccess file.

RewriteCond statements are as they sound - conditional statements for RewriteRules. The basic format for a RewriteCond is RewriteCond test_string cond_pattern. For our purpose, we will set the test_string to be the HTTP_REFERER. If the test string is neither empty nor our own server, then we will serve an alternative (low bandwidth) image, which tells the person who is hotlinking off for stealing our bandwidth.

Here’s how we do that:

RewriteEngine on
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !^http://(www\.)?somesite.com/.*$ [NC]
RewriteRule \.(gif|jpg|png)$ http://www.somesite.com/nasty.gif [R,L]

Here, the RewriteRule will only be performed if all the preceeding RewriteConds are fulfilled. In the second RewriteCond, [NC] simply means “No Case”, so it doesn’t matter whether the domain name was written in upper case, lower case or a mixture of the two. So, any requests for gif, jpg or png files from referers other than somesite.com will result in your “nasty” image being shown instead.

The [R,L] in the RewriteRule simply means “Redirect, Last”. So, the RewriteRule will visibly redirect output to “nasty.gif” and no more RewriteRules will be performed on this URL.

If you simply don’t want the hot linkers to see any image at all when they hot link to your images, then simply change the final line to RewriteRule \.(gif|jpg|png)$ - [F]. The - means “don’t rewrite the requested URL”, and the [F] means “Forbidden”. So, the hot linker will get a “403 Forbidden message”, and you don’t end up wasting your bandwidth.

Conclusion

mod_rewrite is an incredibly handy tool to have in your arsenal. This article only scratched the surface of what is possible with mod_rewrite, but should have given you enough information to go out and start mod_rewriting history yourself!