add_rewrite_rule() │ WP 2.1.0

Here is a simple example of how to register a new rewrite rule, and pass it off to a PHP file for rendering:

1 - Setup a rule:

add_action( 'init',  function() {
	add_rewrite_rule( 'myparamname/([a-z0-9-]+)[/]?$', 'index.php?myparamname=$matches[1]', 'top' );
} );

2 - Flush permalinks. Go to WP Admin > Settings > Permalinks > Save. This doesn’t happen automatically after you add this code

3 - Whitelist the query param:

add_filter( 'query_vars', function( $query_vars ) {
	$query_vars[] = 'myparamname';
	return $query_vars;
} );

4 - Add a handler to send it off to a template file:

add_action( 'template_include', function( $template ) {

	if ( get_query_var( 'myparamname' ) ) {
		return get_template_directory() . '/template-name.php'; 
	}

	return $template;

} );

We create a "nutrition" page (nutrition, ID=12) to display various nutrition information on it. Then, this page uses a separate template. The request variables food and variety are passed to the page.

Let's replace the ugly URL: /nutrition?food=mush&variety=semolina with the pretty ones: /nutrition/mush/semolina

add_action( 'init', 'do_rewrite' );

function do_rewrite(){

	add_rewrite_rule( '^(nutrition)/([^/]*)/([^/]*)/?', 'index.php?pagename=$matches[1]&food=$matches[2]&variety=$matches[3]', 'top' );
	// you need to specify ?p=123 if such a rule is created for post 123
	// the first parameter for posts: p or name, for pages: page_id or pagename

	// tell WP that there are new query parameters
	add_filter( 'query_vars', function( $vars ){
		$vars[] = 'food';
		$vars[] = 'variety';
		return $vars;
	} );
}

Now you need to update the rewrite rules, to do this, just visit the Settings > Permalinks page - the rules will update in the WP options.

You can get the parameters in the page code through the get_query_var() function. For example, link: /nutrition/mush/semolina

echo get_query_var('food');    // mush
echo get_query_var('variety'); // semolina

Another example of creating an Friendly URL for a static page. First the code:

// rewrite rule for the sitemap page
add_action( 'init', 'rewrite_rule_my' );

function rewrite_rule_my(){
	add_rewrite_rule( '^(sitemap)/([^/]*)/?', 'index.php?pagename=$matches[1]&pagetype=$matches[2]', 'top' );

	add_rewrite_tag( '%pagetype%', '([^&]+)' );
}

Here we created the Friendly URL for the permanent page /sitemap?pagename=value (pagename is a query variable). After placing this code in theme functions.php, the Friendly URL of the following form will start working: /sitemap/value and in PHP on this page, you can use the variable $wp_query->query_vars['pagetype'], which will contain the value value. Or you can get the value like this: get_query_var('pagetype').

In the parameter pagename we need to pass the whole link, not just the page slug. Suppose the URL of our child page is /parent/slug, then:

add_rewrite_rule( '^(parent/slug)/([^/]*)', 'index.php?pagename=$matches[1]&foo=$matches[2]', 'top' );

That is, in the regular expression you should write (parent/slug) instead of parent/(slug).

WP will now begin to understand URLs of the form /parent/slug/value, where value will be available through get_query_var('foo').

Using rewrite rules to redirect to scripts other than index.php.

The $redirect argument works differently when redirecting to a non index.php PHP script. In this case, WordPress passes these redirects to .htaccess instead of handling them itself.

For this reason, variables should be written as $1 instead of $matches[1].

add_action( 'init', 'custom_rewrite_rule', 10, 0 );

function custom_rewrite_rule() {
	add_rewrite_rule( 'nutrition/([^/]*)/([^/]*)/?', 'path/to/script.php?food=&variety=', 'top' );
}

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteRule .* - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}]
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteRule ^nutrition/([^/]*)/([^/]*)/? /path/to/script.php?food=&variety= [QSA,L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress

Another example: Friendly login link

If we type http://example.com/login into the browser line, we open the login page http://example.com/wp-login.php (the URL will not change - there will be no redirect):

// Let's make http://site.com/login count as a request for http://site.com/wp-login.php
add_action( 'init', 'wp_pretty_login' );

function wp_pretty_login() {
	add_rewrite_rule( 'login$', 'wp-login.php', 'top' );
}

Get in .htaccess:

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteRule .* - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}]
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteRule ^login$ /wp-login.php [QSA,L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress