Beckham sets up Galaxy winner
Two magnificent strikes from Teal Bunbury put Sporting ahead in Vancouver before the 21-year-old US international set up Kei Kamara for a commanding 3-0 lead in the 62nd minute.
But the expansion Whitecaps roared back with a close range effort from Atiba Harris and two dramatic injury-time goals from Brazilian forward Camilo Sanvezzo.
The Galaxy, who had a slow start to the season, earned three points from a tough battle with Philadelphia, scoring the game's lone goal in the 33rd minute when Beckham whipped in a one of his trademark free-kicks from deep on the right that Brazilian defender Leonardo headed home.
L.A. were forced to play most of the second half with 10 men after English-born Trinidad international Chris Birchall was sent off in the 53rd minute.
The New York Red Bulls, with former France and Arsenal striker Thierry Henry back from injury, welcomed newly-acquired Canada international Dwayne De Rosario to the line-up for their home game against Houston.
The midfielder, acquired on Friday from Toronto, came off the bench and had an instant impact setting up their opening goal in the 47th minute for pacey Jamaican Dane Richards.
But Houston fought back just three minutes later through Cam Weaver to force a draw for the Dynamo.
The Seattle Sounders remain winless after four games as they were held to a 2-2 draw at San Jose while the expansion Portland Timbers picked up their first point in a 1-1 draw on the road at the undefeated New England Revolution.
Toronto, playing their first game without former captain and fan favourite De Rosario, gave up a goal in the 2nd minute against Chivas USA but managed to battle back for a 1-1 draw.
On Friday, the Columbus Crew sank last year's MLS Cup runners-up FC Dallas 2-0.