Phillips followed up his opening-day double at Falkirk with the only goal from the spot as the Paisley side beat visitors St Johnstone.
Goalkeeper Jacob Chapman was the other Buddies hero, saving a spot-kick from Jamie Gullan in the 48th minute.
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His opposite number, Toby Steward, had a far less positive influence on proceedings when he conceded the 73rd-minute penalty that settled matters.
The Perth side's goalkeeper was unaware of the impending presence of Samuel Ramos as he prepared to pick up Declan John's forward pass and brought down the midfielder when he ran into his path.
Phillips sent Steward the wrong way with the shortest of run-ups to make it two wins from two for Craig McLeish's side.
Both sides had started their Premiership campaigns with victories and there was little to separate them in the first half.
The only real chance fell for Ramos, who headed Chris Mochrie's cross straight at Steward. The goalkeeper pushed the ball up in the air and the visitors survived the resultant goalmouth scramble.
Simo Valakari's side squandered their first serious opportunity, which came after a VAR intervention when referee Ross Hardie ruled Alex Gogic had handled while competing for a throw-in into the box. Chapman dived to his left to stop Gullan's penalty.
The home side soon lost Marcus Fraser to injury with Liam Donnelly coming on at centre-back.
Gullan looked to make amends when he set up Matty Foulds but the left-back shot just wide on the run.
A double substitution almost paid instant dividends for McLeish as Luke Thomas set up Jake Young to bear down on goal from the left flank. The striker appeared to be held by Cheick Diabate as he shot wide of the far post.
St Mirren got their breakthrough after more attacking play down the left flank and they soon survived a scare when Diabate headed off the post following a corner.
The hosts had a series of shots blocked as they tried to put the game out of sight in the closing stages but they were soon caught on the break and Ruari Paton set up Gullan. The former Hibernian striker lacked conviction though and was foiled by a combination of Steward and Ryan Carr.
St Johnstone kept the pressure on during stoppage time but Young forced a save on the break at the other end and the home side saw out the remaining moments in the opposition half.
