You want to integrate SMF with something else. Here's 2 files that will help you accomplish that.
The API will let you control SMF from your other system.
The integration hooks will let you control your other system from SMF
How to use the API:
1. Upload it to your server. Anywhere. Make sure your SMF installation is on the same server. It will seek out your installation and create a settings.txt file with the location of your Settings.php file for it's own future reference (to make it load faster)
2. In your script, include the API file. Include it anywhere you like as long as it gets included before you try to use it's functions.
require_once '/path/to/api/file/smf_2_api.php'
3. Call the functions. They are documented pretty well but if you have questions ask below.
Example:
smfapi_logout($email);
Would log the user out of SMF.
This makes the assumption that you have the users email address in the string $email. You would put this code in your other system just before or after the user is logged out there, so now they are logged out together.
4. Repeat for all the other functions you need to use.
How to use the integration hooks:
1. Make it easy on yourself and download the integration hooks skeleton file here. It has been written for maximum ease of use.
2. Upload the skeleton file onto your server. Anywhere. Make sure your SMF installation is on the same server.
3. Open the file index.php in your SMF root directory. This is SMF's 'main' index.php file. At the very top, include the hooks file:
[code]require_once '/path/to/hooks/file/smf_2_integration_hooks.php'
[/code]
4. Write the code inside the functions that will manipulate your other system. Taking logout as an example, inside function smf_logout_function($memberName) you might put:
function smf_logout_function($memberName)
{
$sessionData = smf_session_save_function(); //save and close the SMF session
require_once('other/system/file.php');
other_system_function_logout($memberName);
smf_session_restore_function($sessionData); //load the SMF session and put session data back
}
5. Repeat this process for all the hooks you want to use.
Hint: For the hooks you can get SMF variables in the functions by declaring them as global.
function smf_logout_function($memberName)
{
global $user_info, $context;
//now you have the user info and context variables SMF has available to your function
}
Hint: For the hooks, if you aren't sure what variables are being passed to it, do something like this:
function smf_logout_function($memberName)
{
var_dump($memberName);
exit();
}
Then log out of SMF and you will see what information SMF is passing to the hook.
Questions, comments and suggestions welcome. Please post your finished hook files for other systems too and it will help others using that system to integrate
edit 11/1/11 updated integration hooks file to make fixes to the save and restore functions
NEW - I made a 'REST' API for SMF that you can download and use. Check out it's thread here:
http://www.simplemachines.org/community/index.php?topic=458832.0It let's you use all the SSI functions, all the API functions, a couple other things like post and pm, it's easy to extend, and you do it all from a different domain than your SMF install (or same if you like)

edit 2/29/12 updated API to version 0.1.2 to fix some bugs and make it work with magic quotes hopefully