Edit Dec 2021: (version 2021.2.3): option is now called "Search for variable's definition outside the current file"
Edit Jan 2021: (version 2020.1.1): option is now called "Report in files containing include statements"
Make sure the "Ignore 'include' and 'require' statements" option is unchecked in the Editor > Inspections > Undefined Variable
. After doing this you will have auto-complete variables.
Annoyingly this doesn't give you type information about those variables, even if they're defined with accompanying PHPDoc.