I have benn using PHPStorm just recently, I tried searching on how to do this but I couldn't find it.
It would be great if you could drag and drop files from the Project Explorer to import.
For exemple on Dragging a image to a HTML file it would create a <img src="..."> with the relative path of the image already filled out. The same for CSS and JS files.
Also when Dragging a PHP file to a PHP file it would create a require of that script.