Graphs don't load when using php 5.6 on Debian stretch.

[yuelu]

I am trying to get weathermap plugin to work on Debian stretch which has php 7.2 installed by default. So I installed php 5.6 on the system. However when I switch cacti to use php 5.6, the graphs don't load, neither does System Utilities-> Technical Support. I don't see any related errors in cacti.log or apache log. Browser console shows Failed to load resource: the server responded with a status of 500 (Internal Server Error). When I switch back to php 7.2, the images load fine. Cacti version is 0.8.8h which is the default for Debian stretch. Anyone has any idea why this is happening? How do I go about trouble-shooting it? Thanks.

[netniV]

Check the apache/ngix error logs when you switch to 5.6, you'll probably find that is recording the issue.

[yuelu]

I am not using nginx. I set apache loglevel to "warn". There is nothing in apache error log.

[netniV]

You should not be getting an error 500 without it appearing in the error logs of apache. Error 500 means internal server error, which Apache should always be recording unless you've managled your logging configuration somehow.