Lahar and Pyroclastic Flow are synonyms
As I understand it, the are not the same. Similar but with differences. A pyroclastic flow is volcanic debris propelled by gases while a lahar is volcanic debris propelled by water. A pyroclastic flow is primarily composed of super-heated volcanic ash emitted at the eruption while a lahar is basically a mudslide comprised of ash, dirt, and water which begins to be created at the eruption but whose effects come later after the eruption. Pyroclastic flows kill through scorching or by toxic gases. Lahars kill by crushing those unable to get away from the cement-like slurry.