I currently have some nightmares with PHP 5.6 (application is a #CodeIgniter2 application and cannot run on newer versions, I guess upgrading CI2 to CI3 + PHP to 7.x will make it more messier than before).
Here is what I get from an AJAX request:
<div style="border:1px solid #990000;padding-left:20px;margin:0 0 10px 0;">
<h4>A PHP Error was encountered</h4>
Also no error is being logged but I enabled it in FPM's php.ini file.