A group of friends who were in treatment with Philip Seymour Hoffman are vowing to stay sober FOR GOOD following his death ... TMZ has learned.

TMZ spoke with a woman in rehab with PSH -- we kept her name confidential for obvious reasons -- and she says a bunch of people in rehab and Narcotics Anonymous with Hoffman made a pact after he died to not only stay sober from now on for themselves ... but also for Philip.

We're told his death was a "pivotal moment in their recovery" ... because they realized no one is invincible ... even rich celebrities.