In the "Quick Start", we used nix-shell to open a subshell with a suitable
environment. For example, it sets a PATH which includes the PHP folder
(/nix/store/03z7c0xrd530lpcc7f57s5mfaqg0fdvj-php-7.2.13/bin).
In PhpStorm, the IDE needs a reference to the PHP interpreter. Pointing it
to /nix/store/03z7... is a bit ugly (and hard to maintain during
upgrades). Instead, you can create a "profile" -- a folder which links to
all the packages from locolamp. In this example, we put all the packages
under /nix/var/nix/profiles/per-user/$USER/locolamp:
$ cd locolamp
$ nix-env -p /nix/var/nix/profiles/per-user/$USER/locolamp -i -f .
Now, to register this copy of PHP in PhpStorm (or any other IDE),
use /nix/var/nix/profiles/per-user/$USER/locolamp/bin/php.
Note: Upgrades won't be automatic; so you may want to periodically reinstall the profile.