The two characters that were destroying our CodeIgniter sessions

A CodeIgniter project I was working on for work had a strange bug where users were getting mysteriously logged out. After hours of debugging I was able to narrow it down to its minimal reproduceable form:

$this->session->set_userdata('example_session_value', array('\/'));

Running that line of code will render the current session irretrievable. We were running version 2.1.3. I also tried the most recent stable version (2.1.4) and the bug persists. However, the current development branch seems immune to the bug.

While I don't exactly understand why it breaks, it has something to do with the escaping CodeIgniter does before it serializes the session array to store it in the database.

Update: (Dec 21st, 2013) After considerable digging my brother and I found the root of the problem. In the days of Magic Quotes a strip_slashes() was needed to strip cookie escapement. It has been removed in development branch of CodeIgniter but still persists in the most recent stable release. We have written up a possible history explaining how this issue persists as part of a related pull request.