Along with the ability to password protect folders on the web server, another major use of the .htaccess file is the ability to 'rewrite' the URL. This will let you create more structured and easy to remember URLs. This module will redirect the user to one page while showing another URL in the address bar.
Application of mod_rewrite
Used in del.icio.us
This method is used to great effect in sites like Wikipedia and Del.icio.us. For example let us take a del.icio.us URL...
http://del.icio.us/binblog/javascript+ajax
This does not mean that there is a folder called binblog in the root of del.icio.us site. Nor does this mean that there is a file with the name 'javascript+ajax'. This trick is done using URL manipulation. In this example, the URL can be split into three parts....
http://del.icio.us/binblog/javascript+ajax
^^^ ^^^ ^^^
Site URL User ID Tags
del.icio.us binblog javascript and ajax
The actual URL being called may be something like...
http://del.icio.us/show_bookmarks.php?format=html&user=binblog&tags=javascript+ajax
So how do the user see one URL and the server use another? That is the subtle art of URL manipulation. Before we see the details of this method, a small warning. You may not be able to get the concept at the first glance - it may be sometime before you understand mod_rewrite completely. So - don't give up, grasshopper. As one person puts it...
Despite the tons of examples and docs, mod_rewrite is voodoo. Damned cool voodoo, but still voodoo.
Brian Moore
Before going any further, let me also warn you that you must know regular expression to understand how this works. OK, now we can go further.
Using mod_rewrite
First we need a regular expression to extract the necessary elements from the URL. We will ignore the site URL(http://del.icio.us/) part. The URL is
http://del.icio.us/binblog/javascript+ajax
The Regualar Expression is...
^([^\/]+)\/([^\/]+)$
The extracted strings will be...
$1 = binblog (First Match)
$2 = javascript+ajax (Second Match)
To make this effect using the mod_rewrite module, open the .htaccess file your favorite editor and type in the following lines...
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteRule ^([^\/]+)\/([^\/]*)$ show_bookmarks.php?format=html&user=$1&tags=$2
</IfModule>
Now to test this, create a file called 'show_bookmarks.php' in the document root of your web server. I hope that I don't have to tell you that you need a LAMP setup to do this. You will need atleast Apache and PHP to do this.
After creating the 'show_bookmarks.php' file, enter the following code into it...
<pre><?php print_r($_GET); ?></pre>
Since this is just a test, I did not bother with all the HTML tags like <html>,<body> etc. But if you are a sticker for the rules, go ahead and make the full document.
Next, open this URL in the browser...
http://localhost/show_bookmarks.php?format=html&user=binblog&tags=javascript+ajax
You should get this result - the contests of the GET request...
Array
(
[format] => html
[user] => binblog
[tags] => javascript ajax
)
Now try it with the URL...
http://localhost/binblog/javascript+ajax
If everything went well, this also should have the same output - that is...
Array
(
[format] => html
[user] => binblog
[tags] => javascript ajax
)
Explanation
IfModule
<IfModule mod_rewrite.c>
This is an if condition - the code inside these tags will only be executed if the mod_rewrite module is loaded with apache.
RewriteEngine
RewriteEngine On
The 'RewriteEngine' directive enables or disables runtime rewriting engine. Here we are turning on the re-write Engine. Use the value 'off' if you want to turn of all rewriting.
RewriteRule
RewriteRule ^([^\/]+)\/([^\/]*)$ show_bookmarks.php?format=html&user=$1&tags=$2
This is the important statement. The proper syntax for RewriteRule directive is given below...
RewriteRule Pattern Substitution
The Pattern is a perl compactable regular expression.
Substitution part of the rewriting rule is the string which replaces the original URL for which Pattern has matched. You can use $n to insert regular expression captured strings - $1 is the first capture, $2 will be the second and so on.
One than one line of RewriteRule can be used. The order of useage is important as the second line will use the result of the first substitution as its input
</IfModule>
End of the if condition we started earlier.
Conditions
Some of you must have already seen a big problem in this approach. To see this problem, create a folder called, say, 'data' in your document root. Now create a file called 'something.txt' in this folder. Then try to access this folder from a browser using the URL.
http://localhost/data/
Now you see the problem, don't you? The above URL will result in the output...
Array
(
[format] => html
[user] => data
[tags] =>
)
Our mod_rewrite rules have captured the URL of a valid file along with the other URLs. To solve this problem, we will use a feature of mod_rewrite called Conditions. Insert these lines in the .htaccess file.
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^([^\/]+)\/([^\/]*)$ show_bookmarks.php?format=html&user=$1&tags=$2
</IfModule>
See the line RewriteCond %{REQUEST_FILENAME} !-d? This will make sure that the requested file is not a directory. The line RewriteCond %{REQUEST_FILENAME} !-f will prevent files from being caught by the rewrite rule. The algorithm of these statements will look something like this...
if( 'Requested Filename' IS NOT Directory ) {
if( 'Requested Filename' IS NOT File ) {
Rewrite the URL.
}
}
I hope you got the logic behind this - it took me a while to understand. Anyway, as I said earlier, don't be dissappointed if you don't get it at the first try - there is a lot of black magic involved.
Now try to access the file we created a little while back...
http://localhost/data/something.txt
You will see that it works perfectly(hopefully). Now try...
http://localhost/data/
Again the folder is being accessed. Now try a URL that must be re-written...
http://localhost/binblog/javascript+ajax
If all goes well, this URL will be caught by our system and will be redirected to the show_bookmarks.php file.
More about mod_rewrite in the next post(mod_rewrite Directives - RewriteCond and RewriteRule).
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