This project was confirmed by Gabe Newell and JJ Abrams at the 2013 DICE Summit when they discussed storytelling in movies and video games.
Quintin Tarantino is a fan of the video game series and wishes to direct a movie at some point
J.J.Abrams mentioned with Gabe Newell at DICE 2013 the possibility of a Half-Life movie.
Gabe Newell confirmed that the movie is still being worked on in a Reddit AMA in 2017.