Basic URI router concept

index.php

<?php
include_once("/path/to/Router.php");

// define a fallback uri
if (!array_key_exists('uri', $_GET))
	$_GET['uri'] = 'home';

$router = new Router();
$router->route($uri);

Router.php

<?php

class Router {
	private $_defaultParameters;
	
	public Router($defaultParameters) {
		$this->_defaultParameters = $defaultParameters;
	}
	
	public function route($uri) {
		// Sanitize the uri
		if (strstr($uri, '?') === FALSE)
			$uri .= '?';
		
		// break URI apart on the first ?-character: left hand is the action (based on file name), right hand is the query string
		list($action, $queryString) = explode('?', $uri, 2);
		
		// parse the query string
		$parameters = parse_str($queryString)
		
		// keep track of missing parameters
		$hasMissingParameters = false;
		
		// fill in missing parameters
		foreach ($this->_defaultParameters as $parameterName => $defaultValue)
			if (!array_key_exists($parameterName, $parameters)) {
				$parameters[$parameterName] = $defaultValue;
				$hasMissingParameters |= true;
			}
			
		if ($hasMissingParameters) {
			$uri = $this->_buildUri($action, $parameters);
			header('Location: ' . $uri);
		} else {
			try {
				$this->_mapAction($action, $parameters);
			} catch (Exception e) {
				// Routing failed. Redirect to a stable page and log the error.
			}
		}
	}
	
	private function _buildUri($action, $parameters) {
		$queryString = http_build_query($parameters);
		
		return $action . '?' . $queryString;
	}
	
	private function _mapAction($action, $parameters) {
		// Use reflection or autoloader to map your action to a specific controller method and supply the necessary parameters.
	}
}