If your project contains directories that are stored in the different Perforce depots, you might need to switch between them. PhpStorm uses P4CONFIG to automatically switch to the respective depot as you use a Perforce-versioned directory.
P4CONFIG is an environment variable that contains the name of P4CONFIG file without a path. If a certain directory is associated with Perforce, PhpStorm seeks for P4CONFIG file in this directory and its parents; if the file is not found, it is sought in the bin directory of PhpStorm installation. When a P4CONFIG file is found, PhpStorm uses the settings contained therein, to connect to the respective Perforce depot.
A sample P4CONFG file might consist of such lines:
P4CLIENT=MyClient P4USER=MySelf P4PORT=ida:3456