Codeigniter on PHP paas

A quick search turned up a couple of options for integrating CI with Monolog, which would likely give you enough flexibility for the log output to find something which works in your environment. The first one I actually looked at in any detail replaced the Log.php file in the system/core directory, but I would recommend renaming the file (and class) to MY_Log and putting it in the application/core directory, instead, for easier long-term maintenance (so you don't accidentally overwrite it when upgrading CI).

Bonfire

Practical CodeIgniter 3
CodeIgniter Testing Guide