PHP showing errors, but display_errors=Off

I'm helping a friend with his first dedicated server (with WHM interface), and I found a weird issue:

He migrated all his websites, and they were working. But he found that mysqli was not working. I looked for a solution, and I finally installed mysqli through EasyApache (reconfiguring Apache). When it finished, the websites were not working and with a weird and undescriptable error (something about headers) and a DEFLATE module error. So I reconfigured EasyApache again, but now with deflate. And then I solved the headers error changing suPHP for CGI. At this moments, websites were running, but with a lot of Strict Standards and Warnings (Joomla websites, mostly). I went to PHP configuration, and disabled errors: display_errors = Off. I restarted Apache, and most Websites worked.

The problem is that some websites (I repeat, Joomla that were working perfectly before) show PHP warnings, notices and strict standards messages, while other no. There are no display_errors directive in /home/[user]/public_html/php.ini, in the general php.ini display_errors = Off, and the codes of the websites don't have anything about error_reporting or ini_set('display_errors'...)

Probably this is not the place to ask, but I don't know what else to look at and I don't know who to ask.
Thank you in advance

 

Thank you for your answers. I was able to solve the problem, but I don't know exactly how. I changed some files, which didn't work, and then I undid the changes, and it worked. So it's solved, but I don't know how.

Thank you anyway!