We recently did a major WordPress site upgrade (3.7.25 to 4.9.4 - yes, probably shouldn't have waited this long). I set up a test environment first and thought there would be no issues. However, upon going live, one of our old plugins didn't work after the upgrade. I did research on the issue and discovered that in 4.9.2 a change was made to always use mysqli by default, and I figured that with this being an older plugin that was probably the issue. A link I found said to add this line in the wp-config.php file:
This solved the issue, everything is working correctly now.
My question is, are there any consequences to using this workaround and enabling the "old" mysql_* functions? Our whole point of upgrading WordPress (and about a year back, our PHP version) was for better security, so I don't want to backtrack and have us be as vulnerable as if we were still on the old version anyway. But at this point we still need this plugin, and updating it is probably not going to happen...