Here’s an easy way to get split panes in PHPStorm without tearing your hair out!

If you want Project and Structure then:

  1. Make sure Project has docked and pinned mode ticked (using the cog).
  2. Make sure Structure has docked, pinned and split mode ticked.

You then need to toggle them both a few times for it to work cmd+1 and cmd+7 should work.