I have been having this problem for some time and hoped version 9 would have the fix, but it does not.
I would like to change the severity and severity color for the inspections analysis warnings of "undefined variables" that appear in the right-hand sidebar above the page scroll. Rather than being a RED error, I would like them to appear as Weak Warnings, but I am unable to make it work.
In the Editor > Inspections preferences, I have located the PHP > Undefined > Undefined variables setting and set the Severity levels to Weak Warning in All Scopes. The inspection is enabled in the global space.
However, undefined PHP variables still cause RED error lines to appear in the scroll bar in the editor.
Am I doing something wrong?