Having two different sessions in same domain

You may be able to use session_set_cookie_params to set the domain and folder for the session to be saved under. IE:

// Used on foo.com/example
session_set_cookie_params(86400, '/example');

// Used on foo.com/bar
session_set_cookie_params(86400, '/bar');

You should call session_name before calling session_start. This sets the name of the cookie used to identify the session (by default this is PHPSESSID).

Use a different name for each application. You shouldn't have to mess with the variables inside the session.

I think it's very important to highlight the potential security implications associated with the solutions provided so far. I have been a web application penetration tester for about 5 years and have developed numerous vulnerable security applications in this time to assist with training of juniors starting out in IT security.

I have just been testing the solutions provided and have noted that none of them prevent access to a session belonging to the neighbouring app. Using different session identifier names with session_name() doesn't prevent users from using the value of these identifiers. PHP doesn't have a segregated storage for each session identifier name. I had two apps using different session names and setting a cookie path for the browser. The following respective Set-Cookie directives were included in HTTP responses:

Set-Cookie: TESTONE=<value one>; path=/testone/

Set-Cookie: TESTTWO=<value two>; path=/testtwo/

If both apps had entirely separate users and someone only had access to the /testtwo/ app, they may be able to access info on the /testone/ app depending on the way in which session parameters were being handled. An example code segment below shows a potential data breach assuming that both apps use a $_SESSION["authenticated"] parameter after successful authentication.

<?php 
    session_name("TESTONE");
    ini_set("session.cookie_path","/testone/");
    session_start();
    if ($_SESSION["authenticated"] == "yes")
        echo $topsecretinfo;
?>

To access this $topsecretinfo one would only need to authenticate on the /testtwo/ application, take the value of their TESTTWO session identifier and use it as the value of the TESTONE session identifier when sending requests to the /testone/ application. PHP's session lookup process does not recognise the name of the session identifier except for parsing the correspoding value. i.e. a session identifier value of "agcy648dja6syd8f93" will return the same session object regardless of the name used to refer to it.