Ken Griffey Jr. went 0-for-3, ending his streak of homers in three consecutive games.
Phelps (1-0), making his first start of the season, allowed only two walks and Casey's fourth inning double.
"He was great," catcher Mark Redmond said. "He was the MVP of that game."
Todd Van Poppel (2-2) was solid in his fourth straight start for Cincinnati.
Armando Benitez finished for his 17th save.
Phillies 7, Mets 4
In New York, first baseman Mike Piazza's throwing error set up Jim Thome's tiebreaking double as Philadelphia made a six-run seventh inning to beat New York.
Philadelphia snapped New York's four-game winning streak and remained in a virtual tie with Florida atop the National League East.
Matt Ginter pitched six shutout innings for the Mets, extending their scoreless streak to 28, their longest since going 31 innings without allowing a run in September 1990.
But an the Mets' overworked bullpen failed to hold a 3-0 lead.
Thome hit a two-run double off Mike Stanton, giving the Phillies a 5-3 lead. John Franco (0-3) took the loss
Roberto Hernandez (1-1) walked home a run in the sixth but struck out Piazza with two outs and the bases loaded.
Astros 7, Cubs 3
In Houston, Jeff Bagwell, Jeff Kent and Lance Berkman each homered off Greg Maddux as Houston overcame the early loss of Andy Pettitte to complete a two-game sweep of Chicago.
Chicago was swept for the first time this season, and it was the first time the Astros had done it against their NL Central rivals since May 2001.
Pettitte left in the fourth inning with discomfort in his left forearm, and the team said he was day-to-day.
Maddux (3-4) was roughed up throughout his five-inning outing against the NL's top offense. He failed to go at least six innings for only the second time this season, giving up five runs on nine hits.
Brad Lidge (1-3) struck out all three batters in the eighth.
Giants 4, Diamondbacks 3
In San Francisco, Barry Bonds hit his second homer in two days and Pedro Feliz hit a go-ahead, two-run double in the eighth as the San Francisco rallied to beat Arizona.
Bonds hit a 2-2 pitch from Casey Daigle to dead center for his 12th home run and career No. 670 leading off the fourth innings to give San Francisco a 2-1 lead.
Felix Rodriguez (2-3) pitched the eighth for the win. Brian Bruney (2-2) walked Michael Tucker and Marquis Grissom in the bottom half, then Randy Choate entered to face Bonds.
The Diamondbacks fell to 12 games under .500 -- their worst record since the club's inaugural season in 1998, when Arizona finished 16 games under at 65-97.
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