Yes. The only ways the Phils could get out from under the contract would be if they could trade Walker (not permissible at this time), or by asking unconditional release waivers on him, and having some other club claim the contract (not likely). If they ask release waivers, and nobody claims him, he becomes a free agent, and the Phils are on the hook for the balance of his contract. And because he's a veteran, they can only ask outright waivers on him with his written consent.
Rosters increase by two (from 26 to 28), but only one of those two can be a pitcher. (Technically, the restriction is that no more than 14 of the 28 players on the active roster can be pitchers, but since the Phils currently carry 13 pitchers...)