This fully depends on how that specific PHEV is set up.
A fairly common older setup is ICEF + MGR, (internal combustion engine front, motor-generator rear). Those can run full EV mode at highway speeds no problem, through you will have ICE kick in when you ask ECM (engine control module) for rapid acceleration (accelerator pedal position high enough for ECM to conclude that you want more power than MGR can provide). Essentially ICEF has a normal gearbox, so it will just switch into neutral and shut down the engine, driving on MGR only.
More modern are on the expired Toyota patent that allows fully combined hybrid drive train in front. This eliminates a lot of complexity (no gear box, no starter motor), but does require the power pack to go into energy re-circulation mode at speeds that exceed efficiency rate of MG2 (power pack has two motor-generators and an internal combustion engine all sitting on the same planetary differential, and MG2's output is connected directly to wheels) Notably this is why gen 4 (iirc) onwards, Toyota hybrids have additional planetary differential between MG2 and wheels to reduce MG2 RPM related to front wheel speed. It makes these hybrids retain full hybrid efficiency with no need to go into re-circulation mode at even higher speeds.
Finally there's the all wheel drive option where it's both power pack in front, and MGR in the rear separated from the power pack. Those can also do EV only at highway speeds with way more power, but at somewhat reduced efficiency in MGR + MG2 EV mode (either with additional clutch between MG2 and rest of power pack, or MG1 just spinning fast enough to maintain ICE at zero RPM).