When installing a new version of PhpStorm I had the issue that
only the settings were imported from the old version but not
the plugins – so I had to install them again.

Plugins are located in ~/Library/Application\ Support/WebIdeXXX
where XXX stands for the PhpStorm version number. Here is an example
where I've updated from PhpStorm 9.5 to 10 (WebIde95 to WebIde100).

# backup just in case
cp -R ~/Library/Application\ Support/WebIde100 ~/Library/Application\ Support/WebIde100_backup
# move plugins over
cp -R ~/Library/Application\ Support/WebIde95/* ~/Library/Application\ Support/WebIde100/
# test it, then remove the backup
rm -fr ~/Library/Application\ Support/WebIde100_backup