First, create PhpStorm's configuration folder
mkdir -p ~/Library/Preferences/WebIde80/
Then copy current configuration
cp /Applications/PhpStorm.app/Contents/bin/idea.properties ~/Library/Preferences/WebIde80/
And lastly, open the file and replace JVMVersion=1.6*
to JVMVersion=1.8*
vim ~/Library/Preferences/WebIde80/