If you have trouble using the default ALT + Button1 click keymap in Linux to add to a multi-cursor selection in PHPStorm or any other JetBrains product, you may be able to adjust the default keymap, but if you are like me, you could just as easily use an easier approach. Just add the "super" or "windows" key to the combination. So, try Super + ALT + Button1. That works for me as the desktop environment doesn't see it as an ALT + Button1 click, but PHPStorm does. Using ALT + button1 click alone causes issues for me because the Cinnamon desktop in LinuxMint already has ALT + Button1 mapped to something else, namely window move. I would change this but I actually like having that available if I need it.