http_build_url in PHP, from the comments at http://www.php.net/manual/en/function.http-build-url.php

http_build_url.php

<?php
	if (!function_exists('http_build_url'))
	{
		define('HTTP_URL_REPLACE', 1);				// Replace every part of the first URL when there's one of the second URL
		define('HTTP_URL_JOIN_PATH', 2);			// Join relative paths
		define('HTTP_URL_JOIN_QUERY', 4);			// Join query strings
		define('HTTP_URL_STRIP_USER', 8);			// Strip any user authentication information
		define('HTTP_URL_STRIP_PASS', 16);			// Strip any password authentication information
		define('HTTP_URL_STRIP_AUTH', 32);			// Strip any authentication information
		define('HTTP_URL_STRIP_PORT', 64);			// Strip explicit port numbers
		define('HTTP_URL_STRIP_PATH', 128);			// Strip complete path
		define('HTTP_URL_STRIP_QUERY', 256);		// Strip query string
		define('HTTP_URL_STRIP_FRAGMENT', 512);		// Strip any fragments (#identifier)
		define('HTTP_URL_STRIP_ALL', 1024);			// Strip anything but scheme and host
		
		// Build an URL
		// The parts of the second URL will be merged into the first according to the flags argument. 
		// 
		// @param	mixed			(Part(s) of) an URL in form of a string or associative array like parse_url() returns
		// @param	mixed			Same as the first argument
		// @param	int				A bitmask of binary or'ed HTTP_URL constants (Optional)HTTP_URL_REPLACE is the default
		// @param	array			If set, it will be filled with the parts of the composed url like parse_url() would return 
		function http_build_url($url, $parts=array(), $flags=HTTP_URL_REPLACE, &$new_url=false)
		{
			$keys = array('user','pass','port','path','query','fragment');
			
			// HTTP_URL_STRIP_ALL becomes all the HTTP_URL_STRIP_Xs
			if ($flags & HTTP_URL_STRIP_ALL)
			{
				$flags |= HTTP_URL_STRIP_USER;
				$flags |= HTTP_URL_STRIP_PASS;
				$flags |= HTTP_URL_STRIP_PORT;
				$flags |= HTTP_URL_STRIP_PATH;
				$flags |= HTTP_URL_STRIP_QUERY;
				$flags |= HTTP_URL_STRIP_FRAGMENT;
			}
			// HTTP_URL_STRIP_AUTH becomes HTTP_URL_STRIP_USER and HTTP_URL_STRIP_PASS
			else if ($flags & HTTP_URL_STRIP_AUTH)
			{
				$flags |= HTTP_URL_STRIP_USER;
				$flags |= HTTP_URL_STRIP_PASS;
			}
			
			// Parse the original URL
			$parse_url = parse_url($url);
			
			// Scheme and Host are always replaced
			if (isset($parts['scheme']))
				$parse_url['scheme'] = $parts['scheme'];
			if (isset($parts['host']))
				$parse_url['host'] = $parts['host'];
			
			// (If applicable) Replace the original URL with it's new parts
			if ($flags & HTTP_URL_REPLACE)
			{
				foreach ($keys as $key)
				{
					if (isset($parts[$key]))
						$parse_url[$key] = $parts[$key];
				}
			}
			else
			{
				// Join the original URL path with the new path
				if (isset($parts['path']) && ($flags & HTTP_URL_JOIN_PATH))
				{
					if (isset($parse_url['path']))
						$parse_url['path'] = rtrim(str_replace(basename($parse_url['path']), '', $parse_url['path']), '/') . '/' . ltrim($parts['path'], '/');
					else
						$parse_url['path'] = $parts['path'];
				}
				
				// Join the original query string with the new query string
				if (isset($parts['query']) && ($flags & HTTP_URL_JOIN_QUERY))
				{
					if (isset($parse_url['query']))
						$parse_url['query'] .= '&' . $parts['query'];
					else
						$parse_url['query'] = $parts['query'];
				}
			}
				
			// Strips all the applicable sections of the URL
			// Note: Scheme and Host are never stripped
			foreach ($keys as $key)
			{
				if ($flags & (int)constant('HTTP_URL_STRIP_' . strtoupper($key)))
					unset($parse_url[$key]);
			}
			
			
			$new_url = $parse_url;
			
			return 
				 ((isset($parse_url['scheme'])) ? $parse_url['scheme'] . '://' : '')
				.((isset($parse_url['user'])) ? $parse_url['user'] . ((isset($parse_url['pass'])) ? ':' . $parse_url['pass'] : '') .'@' : '')
				.((isset($parse_url['host'])) ? $parse_url['host'] : '')
				.((isset($parse_url['port'])) ? ':' . $parse_url['port'] : '')
				.((isset($parse_url['path'])) ? $parse_url['path'] : '')
				.((isset($parse_url['query'])) ? '?' . $parse_url['query'] : '')
				.((isset($parse_url['fragment'])) ? '#' . $parse_url['fragment'] : '')
			;
		}
	}