Diff in PhpStorm

It can be done only externally:

You can also open the difference viewer without running PhpStorm. This is done through the following command:

<path to PhpStorm executable file> diff <path_1> <path_2>

where path_1 and path_2 are paths to the files in question, which can be of various types, including jar.

From inside PhpStorm you can only compare files that are available in the project.

Currently, the easiest way that I found to compare a project file with a non-project file is by copying the external file to the clipboard and clicking on "View-> Compare with clipboard".