To understand what a router does, you must first understand what a rewrite engine is. From the Wikipedia article (emphasis mine):
A rewrite engine is software that modifies a web URL's appearance (URL rewriting). Rewritten URLs (sometimes known as short, fancy URLs, or search engine friendly - SEF) are used to provide shorter and more relevant-looking links to web pages. The technique adds a degree of separation between the files used to generate a web page and the URL that is presented to the World.
When a rewrite engine is used you don't have an 1:1 correlation between the URL and a PHP script. An example from the same article:
can be rewritten as:
There are various benefits to using the technique. Since PHP is usually tightly coupled with Apache, the most commonly used rewrite engine is Apache's mod_rewrite.
If you want rewritten URLs, you need some kind of routing, as routing is the process of taking the URL, braking it into components and deciding what is the actual script to call. The documentation page for the standard router of the Zend Framework explains the process as:
Routing is the process of taking a URI endpoint (that part of the URI which comes after the base URL) and decomposing it into parameters to determine which module, controller, and action of that controller should receive the request.
Most PHP frameworks nowadays are based on the MVC pattern, and on an MVC framework the process goes something like this*:
- Browser is pointed to a URL,
- Web server catches request and forwards it to a common entry point, usually an
index.php gets the URL and starts the routing process.
- URL is decomposed into parameters, where the first is the controller, second is the action method, and the rest are considered dynamic parameters,
- If a controller class matching the first parameter exists, an controller object is instantiated,
- The action method which is usually a function of the controller object is called and its return is what actually returned to the browser.
Matching parameters to controllers and methods usually employs matching via regular expressions to be able to handle complex and dynamic routing patterns, known as routes. Good examples of routes can be found on CodeIgniter's URI Routing documentation page:
$route['journals'] = "blogs";
$route['blog/joe'] = "blogs/users/34";
$route['product/(:any)'] = "catalog/product_lookup";
$route['product/(:num)'] = "catalog/product_lookup_by_id/$1";
$route array holds the patterns as keys and the resulting actions as values in
A very nice tutorial that will help you write your own simple router and better understand the concepts involved, is PHP Regular Expresssion URL Router on Query7.
*This is not intended as a description of the canonical process, just an oversimplified explanation.