A 2-1 victory over the Chicago Fire saw Philadelphia push three points clear of Toronto in the Eastern Conference, and for the Supporters' Shield, which it is bidding to win for the first time.
Kacper Przybylko opened the scoring from the penalty spot approaching the half-hour mark, after a handball in the area.
Chicago's chances of a comeback were dealt a blow nine minutes before half-time, Francisco Calvo sent off after a tackle on Alejandro Bedoya.
However, it managed to find an equaliser prior to the break as Robert Beric headed in from close range.
But Philadelphia scored what proved to be the winner in the 65th minute, when Cory Burke headed in a cross from Olivier Mbaizo.
Union became the first team to win its first eight home matches of an MLS season since San Jose in 2002, when the Earthquakes claimed their first 10.
The Timbers clinched a spot in the play-offs thanks to a 5-2 thrashing of the struggling LA Galaxy.
Jaroslaw Niezgoda scored a brace for Portland, who were 3-0 up after 30 minutes following Diego Valeri's penalty.
Eryk Williamson and Andy Polo also netted as the Timbers thrashed the Galaxy, who are bottom of the Western Conference.
Portland became the first team with multiple five-goal games against an opponent in a single MLS season since the Fire against the New York Red Bulls in 2008, having thrashed the Galaxy 6-3 earlier this month.
Chris Wondolowski scored a brace as the Earthquakes enjoyed a 2-0 win over Real Salt Lake.
It was his 24th multi-goal game in MLS. Only Landon Donovan (32) has more.
New York Red Bulls left it late to edge New England Revolution 1-0 and reach the play-offs, Sporting Kansas City overcame Cincinnati 1-0 and Orlando City thrashed Atlanta United 4-1.
New York City upset Toronto 1-0, Minnesota United beat Colorado Rapids 2-1, DC United was too good for Columbus Crew 1-0, Dallas claimed a 2-1 win over Inter Miami and Los Angeles FC was a 2-1 victor over Houston Dynamo to clinch its play-off place.