Codeigniter 3 consecutive transactions not working
Bug disappeared when upgrading my version of php from 5.5.10 to 5.6.10
In order to try to get a more accurate estimate of the version fixing the bug I tried PHP 5.5.22 which works (couldn't get anything between that and 5.5.10 for MAMP).
I checked the PHP ChangeLog and noticed this for version 5.5.12:
mysqli: Fixed problem in mysqli_commit()/mysqli_rollback() with second parameter (extra comma) and third parameters (lack of escaping).
Which doesn't link to any further details but is the only thing in the change logs that seems related to my issue and that matches the versions for which I have the bug (before 5.5.10 & after 5.5.22).