I created a lamp stacked vagrant box with PHP 5.6 but my sessions didn't want to start. After some digging i noticed my session.save_path was linking too a folder that was "mounted" by vagrant. This caused PHP to fail on writing sessions and caused symfony to not be able to use sessions.
EDIT: This only seems to ocure on Mac
Step 1: Look for you session.save_path in phpinfo();
If the local value points to a folder that is "mounted" by vagrant this will cause issues. In my case /vagrant/ is my mount folder.
You can also use the following test case to reproduce your problem:
Create a test.php in your project:
<?php session_save_path('/vagrant/sessions'); session_start(); file_put_contents('/vagrant/sessions/file.txt', 'TEST');
If you run this command line you'll see the file.txt is created successfully and your session create gives the following warning:
PHP Warning: Unknown: Failed to write session data (files). Please verify that the current setting of session.save_path is correct (/vagrant/sessions) in Unknown on line 0
Step 2: Fixing the problem Create a folder for your sessions to be written in:
mkdir -p /var/lib/php5/<folder> chown vagrant:vagrant /var/lib/php5/<folder>
Add this line to your config to tell symfony to write you session in the default session path instead of the 1 you saw in Step 1.
framework: session: handler_id: session.handler.native_file save_path: /var/lib/php5/<folder>
Step 3: Enjoy