Use PHPStorm/WebStorm for git diff and merge tools

.gitconfig

# ~/.gitconfig
# Add this to your global git configuration file
# Change phpstorm to webstorm, if you use that.

# Diff and merge tool changes
# Run `git difftool <directory/file>...` or `git mergetool <directory/file>...`
[merge]
	tool = phpstorm
[diff]
	tool = phpstorm
[difftool]
; This setting is optional, but prevents an annoying prompt between diffing files.
	prompt = false
[difftool "phpstorm"]
; Putting this path here doesn't work. I would love to know why.
;	path = /Applications/PHPStorm.app/Contents/MacOS
	cmd = webide diff $(cd $(dirname "$LOCAL") && pwd)/$(basename "$LOCAL") $(cd $(dirname "$REMOTE") && pwd)/$(basename "$REMOTE")
	trustExitCode = true
[mergetool "phpstorm"]
	cmd = webide merge $(cd $(dirname "$LOCAL") && pwd)/$(basename "$LOCAL") $(cd $(dirname "$REMOTE") && pwd)/$(basename "$REMOTE") $(cd $(dirname "$BASE") && pwd)/$(basename "$BASE") $(cd $(dirname "$MERGED") && pwd)/$(basename "$MERGED")
	trustExitCode = true

.profile

# Add this to your .profile, .zshrc, .bashrc, etc.

# Add PHPStorm's bin directory to your path.
export PATH="/Applications/PHPStorm.app/Contents/MacOS:$PATH"

Thank you for putting me on the right path. This also works on Windows if the git commands for diffing and merging are executed within Git Bash (the sub-shells and pwd calls work fine under cygwin). My config, using the 64 bit PhpStorm binary as the diff and merge tools with SourceTree:

[merge]
    tool = sourcetree
[diff]
    tool = sourcetree   
[difftool]
    prompt = false
[difftool "sourcetree"]
    cmd = 'C:/Program Files (x86)/JetBrains/PhpStorm 8.0.1/bin/PhpStorm64.exe' diff $(cd $(dirname $LOCAL) && pwd)/$(basename $LOCAL) $(cd $(dirname $REMOTE) && pwd)/$(basename $REMOTE)
    trustExitCode = true
[mergetool "sourcetree"]
    cmd = 'C:/Program Files (x86)/JetBrains/PhpStorm 8.0.1/bin/PhpStorm64.exe' merge $(cd $(dirname $LOCAL) && pwd)/$(basename $LOCAL) $(cd $(dirname $REMOTE) && pwd)/$(basename $REMOTE) $(cd $(dirname $BASE) && pwd)/$(basename $BASE) $(cd $(dirname $MERGED) && pwd)/$(basename $MERGED)
    trustExitCode = true

You don't need all of that $(blah blah) any more. Newer versions of PHPStorm and WebStorm have a command line tool, called pstorm or wstorm. So the git config is just:

[diff]
  tool = pstorm
[difftool]
  prompt = false
[difftool.pstorm]
  cmd = /usr/local/bin/pstorm diff "$LOCAL" "$REMOTE"
[merge]
  tool = pstorm
[mergetool.pstorm]
  cmd = /usr/local/bin/pstorm merge "$LOCAL" "$REMOTE" "$BASE" "$MERGED"

Or:

[diff]
  tool = wstorm
[difftool]
  prompt = false
[difftool.wstorm]
  cmd = /usr/local/bin/wstorm diff "$LOCAL" "$REMOTE"
[merge]
  tool = wstorm
[mergetool.wstorm]
  cmd = /usr/local/bin/wstorm merge "$LOCAL" "$REMOTE" "$BASE" "$MERGED"

John. Thank you for your comment. I'm a PHPStorm user. I popped the parameters into .gitconfig and, voila, worked like a charm.

For anyone interested, I used the following command to invoke PHPStorm so I could compare a file across two branches.
git difftool <branch1> <branch2> -- <path/filename>

And for those who need to see a more concrete example:
git difftool new-feature origin/master -- js/broadcast.js

Does anyone know how to configure phpstorm's diff or merge tool to launch in the foreground when issuing a git difftool or git mergetool? I find the background launch inconvenient.

great!

but it doesn't show me any output to me, neither error.

does it conflict with other open PHPStorm istances?
Well, i was expected that a window will pop up to resolve the conflicts, independtly. But nothing shows up and git ask me if the conflict was resolved.

Stopped working for me all the sudden. Now "git mergetool /somefile/..." gives me "No IDE instance has been found. New one will be started."

Like @raffaelo I used to have a mergetool gui pop up so I could visually merge what I wanted was best tool around.

JetBrains support supplied this to fix it: https://confluence.jetbrains.com/display/PhpStorm/PhpStorm+Early+Access+Program

Verified working: PhpStorm-EAP-162.1447.5.dmg

None of the solution worked for me Webstorm 2016.2, unless I set
prompt = true
Then even the handy solution of JohnAlbin works.

If prompt = false, only the first file in the list is opened correctly, then all the other files are not found, prompting:
Error showing diff: Can't find file /tmp/Us1QSf_myfile.js

@thoma5B You just saved me some headaches with that prompt, thanks mate.

If you can not find wstorm, open WebStorm, and go to Tools in menu bar then click Command-line Laucher. My WebStorm version is 2016.2.3.

BTW, I can directly use the solution of @JohnAlbin. Thanks.

• You can also configure it with commands like git config --global mergetool.pstorm.trustExitCode true.
• You can use difftool --dir-diff, but currently tmp-files may be instantly removed, so I appended my difftool.pstorm.cmd with ; read and press Enter after diff completion.

FYI: @zored I had to use && read instead on my system, in order for it to not delete the files before I'm done with them.
But that way, it works (for me too). Great work around, though! 👍 😉

P.S.: adding an additional --symlinks flag to the difftool --dir-diff is also a nice "upgrade"...