The man fondly called "The Bull" by friends scored on a penalty kick in the 81st minute to deliver the game’s lone goal and give the hosts their second straight win in three games.
"This is the first time I’ve actually scored with my foot in an international competition," said the 23-year-old team captain, whose previous goals were through headers.
In all, the Philippines now has a total of six points, just one shy of leader Laos, which was battling Brunei at press time.
The top two in this five-team Asean Football Federation event, hosted by the Negros Occidental Football Association and sponsored by Pepsi, Mizuno, Gatorade, PLDT, Premier Water, Petron, Palmas del Mar and GMA Radio Network, advance to the tournament proper in January.
To be largely blamed for Cambodia’s defeat was goalkeeper Ouk Mic, who fouled a cutting Emilio "Chipi" Caligdong trying to ram home a James Younghusband pass.
Borromeo, who ceded Phil Younghusband a penalty kick try in the Philippines’ 7-0 win over Timor Leste last Tuesday, won’t be denied this time, calmly sweeping the ball past Mic’s right.
"I was not even thinking about it. I just hit it there," said Borromeo, a veteran of the National Under-23 and 2005 Manila SEA Games teams.
National coach Aris Caslib said the game turned out to be the tough one he expected and credited his players’ fine defensive job.
The Philippines takes on Brunei at tournament’s end Monday, with another victory giving it its highest win total in an international competition in post-war era.