Tony Mowbray's sharpshooters had failed to find the net against struggling Wednesday until the 34-year-old striker steamed in to meet substitute James Morrison's cross to head home.
Wednesday, unbeaten in three games but still in deep trouble, almost caused an early shock as full-back Tommy Spurr cut inside to drill in a 20-yards strike which grabbed by the diving Dean Kiely.
West Brom, whose disastrous run of three wins from nine league games has seen them slip from an automatic promotion spot, almost found a way through when skipper Jonathan Greening picking out centre-back Leon Barnett, whose crisp header was snatched from under the bar by Lee Grant.
Winger Chris Brunt, a Wednesday favourite until his £3million move to The Hawthorns in the summer, had a teasing cross saved at the second attempt from Grant before the Wednesday keeper pulled off a stunning stop on 24 minutes.
Midfielder Robert Koren lashed in a strike from the edge of the area and Grant leapt to his right to push the ball away.
The Hillsborough side had to rely on the hard-working Enoch Showunmi, whose darting runs caused problems for the West Brom back four.
But the on-loan Bristol City striker was at fault minutes before the interval after heading Brunt's corner straight to the lurking Paul Robinson, who should have scored but pulled his shot wide from 12 yards out.
Spurr saved Wednesday with a well-timed sliding tackle as Luke Moore cut in from the left.
Workhorse Showunmi went on another long run but was stopped by a combination of Martin Albrechtsen and Kiely, who deflected the ball round the post.
The game erupted in the 78th minute when Greening and Jermaine Johnson tussled. The Wednesday player thrust an elbow at the ex-Manchester United starlet as he walked away.
But it was Phillips who had the last word after finding the back of the net in the fourth minute of stoppage time.