You presumably used the wrong screwdriver to begin with. Most people should lock up their "small" (#1) Phillips in a vault and never touch it again until they're doing work on small electronics. They're almost never the right size for household hardware. #2 is what you need for 99% of home improvement work, and 0.9% of the time you need #3.
Or your screwdriver is in poor condition. Screwdrivers wear out, especially if they're misused in the wrong size screws or as drilling or prying tools. Phillips screwdrivers can't be easily repaired, so replace them.
Or, you didn't apply force parallel to the screw. Phillips is a convenient design, but it requires good technique. Press straight, press hard, at least until the screw breaks free.
Every homeowner and DIY mechanic should own a mechanical impact driver. They're a tool with screwdriver bits and a stricken handle. They allow directed force to break sticky screws free.
Or, a locking (Vice-Grip) pliers can do the job, if there's clearance to get the jaws on the screw sideways, with the teeth parallel to the screw shaft. Clamping it on end is less effective.
As a last resort, use a rotary tool (Dremel) to cut a clean slot in the screw and use a large flat-blade screwdriver to remove the screws.