Mel Gibson Celebrates I'M 5 YEARS SOBER!

8/1/2011 1:18 PM PDT

Mel Gibson -- The 5-Year Sobriety Celebration

It's been FIVE YEARS since Mel Gibson's infamous 2006 DUI arrest -- and sources close to the actor tell TMZ, he's been off the sauce ever since ... and celebrated the sobriety milestone with friends last week.

Mel was arrested on July 28, 2006 -- and from that day forward ... we're told he hasn't taken a sip of booze.

Mel and some buddies commemorated the occasion last Friday -- and the actor is telling friends ... even though it's been a rough couple years, he's ultimately grateful for his arrest that fateful night.

Time really flies.