From the Menu of PHPStorm Choose Navigate -> SHIFT0 or use the shortcut ALT+SHIFT+O or CMD+SHIFT+O or CTRL+SHIFT+N (as per the edit). Type the required file name you want to search. Done.
Another and easy way would be to press double Shift on Linux or Windows. It would open a search bar and from there select file. More over exact short cut can be found(as told above) by Nagivigation>Find.