http - PHP_SELF vs PATH_INFO vs SCRIPT_NAME vs REQUEST_URI

Question

I am building a PHP application in CodeIgniter. CodeIgniter sends all requests to the main controller: index.php. However, I don't like to see index.php in the URI. For example, http://www.example.com/faq/whatever will route to http://www.example.com/index.php/faq/whatever. I need a reliable way for a script to know what it's address is, so it will know what to do with the navigation. I've used mod_rewrite, as per CodeIgniter documentation.

The rule is as follows:

RewriteEngine on
RewriteCond $1 !^(images|inc|favicon.ico|index.php|robots.txt)
RewriteRule ^(.*)$ /index.php/$1 [L] 

Normally, I would just check php_self, but in this case it's always index.php. I can get it from REQUEST_URI, PATH_INFO, etc., but I'm trying to decide which will be most reliable. Does anyone know (or know where to find) the real difference between PHP_SELF, PATH_INFO, SCRIPT_NAME, and REQUEST_URI? Thanks for your help!

Note: I've had to add spaces, as SO sees the underscore, and makes it italic for some reason.

Updated: Fixed the spaces.

Answer 1

Some practical examples of the differences between these variables:
Example 1. PHP_SELF is different from SCRIPT_NAME only when requested url is in form:
http://example.com/test.php/foo/bar

[PHP_SELF] => /test.php/foo/bar
[SCRIPT_NAME] => /test.php

(this seems to be the only case when PATH_INFO contains sensible information [PATH_INFO] => /foo/bar) Note: this used to be different in some older PHP versions (<= 5.0 ?).

Example 2. REQUEST_URI is different from SCRIPT_NAME when a non-empty query string is entered:
http://example.com/test.php?foo=bar

[SCRIPT_NAME] => /test.php
[REQUEST_URI] => /test.php?foo=bar

Example 3. REQUEST_URI is different from SCRIPT_NAME when server-side redirecton is in effect (for example mod_rewrite on apache):

http://example.com/test.php

[REQUEST_URI] => /test.php
[SCRIPT_NAME] => /test2.php

Example 4. REQUEST_URI is different from SCRIPT_NAME when handling HTTP errors with scripts.
Using apache directive ErrorDocument 404 /404error.php
http://example.com/test.php

[REQUEST_URI] => /test.php
[SCRIPT_NAME] => /404error.php

On IIS server using custom error pages
http://example.com/test.php

[SCRIPT_NAME] => /404error.php
[REQUEST_URI] => /404error.php?404;http://example.com/test.php