htaccess redirect based on query parameter value

Question

My application used to do language switching using a lang.php file. A URL to redirect to English for /path/to/page.php?foo=bar would look like this:

/path/to/lang.php?lang=en-CA&uri=%2Fpath%2Fto%2Fpage.php%3Ffoo%3Dbar

Recently there has been some changes to accept a lang query parameter on all pages. So this URL is nicer:

/path/to/page.php?foo=bar&lang=en-CA

I'd like to be able to add a .htaccess file to all the locations that I have a lang.php file in order to keep the existing URL working without the lang.php file. Is this possible?

The RewriteRules must be completely relative to lang.php as the application is running on different hostnames and paths.

I took a stab at it based on the answer here but this is giving me a 404:

RewriteCond %{REQUEST_FILENAME} lang.php$
RewriteCond %{QUERY_STRING} (?:^|&)uri=([^&]+) [NC]
RewriteRule lang.php - [E=LANG_REDIR_URI:%1]

RewriteCond %{REQUEST_FILENAME} lang.php$
RewriteCond %{QUERY_STRING} (?:^|&)lang=([^&]+) [NC]
RewriteRule lang.php - [E=LANG_REDIR_LANG:%1]

RewriteCond %{REQUEST_FILENAME} lang.php$
RewriteRule . %{LANG_REDIR_URI}e [L,R=temporary]

The uri parameter may already have query parameters (as in the examble above), and the code should not make the application vulnerable to an open redirect vulnerability.

Different cases:

• URI with query string, lang first: /foo/bar/lang.php?lang=en-CA&uri=%2Ffoo%2Fbar%2Fpage.php%3Fid%3D123 should redirect to /foo/bar/page.php?id=123&lang=en-CA
• URI without query string, lang first: /foo/bar/lang.php?lang=en-CA&uri=%2Ffoo%2Fbar%2Fpage.php should redirect to /foo/bar/page.php?lang=en-CA
• URI with query string, uri first: /foo/bar/lang.php?uri=%2Ffoo%2Fbar%2Fpage.php%3Fid%3D123&lang=en-CA should redirect to /foo/bar/page.php?id=123&lang=en-CA
• URI without query string, uri first: /foo/bar/lang.php?uri=%2Ffoo%2Fbar%2Fpage.php&lang=en-CA should redirect to /foo/bar/page.php?lang=en-CA

The order of the query parameters do not matter in the redirect target.

UPDATE

After the initial answer provided at https://stackoverflow.com/a/45732670/404623, I've tried the following .htaccess rules:

# This only works for uri values without query strings

RewriteCond %{QUERY_STRING} ^(lang=[^&]+)&uri=([^&]+) [NC]
RewriteRule lang\.php$ /%2\?%1 [L,NE,R=temporary]
RewriteCond %{QUERY_STRING} ^uri=([^&]+)&(lang=[^&]+) [NC]
RewriteRule lang\.php$ /%1\?%2 [L,NE,R=temporary]

# This only works for uri values with query strings

RewriteCond %{QUERY_STRING} ^(lang=[^&]+)&uri=([^&]+) [NC]
RewriteRule lang\.php$ /%2&%1? [L,NE,R=temporary]
RewriteCond %{QUERY_STRING} ^uri=([^&]+)&(lang=[^&]+) [NC]
RewriteRule lang\.php$ /%1&%2? [L,NE,R=temporary]

Answer 1

You can use this rule in your site root .htaccess just below RewriteEngine On line:

RewriteEngine On

# uri with query string    
RewriteCond %{QUERY_STRING}     (?:^|&)(lang=[^&]+) [NC]
RewriteCond %1##%{QUERY_STRING} (.*)##(?:.*&)?uri=([^?]+)(?:\?|%3F)([^&]+) [NC]
RewriteRule lang\.php$ /%2?%1&%3 [L,NE,R=302]

# uri without query string    
RewriteCond %{QUERY_STRING}     (?:^|&)(lang=[^&]+) [NC]
RewriteCond %1##%{QUERY_STRING} (.*)##(?:.*&)?uri=([^?]+) [NC]
RewriteRule lang\.php$ /%2?%1 [L,NE,R=302]