- He was a surgeon on the U.S. Navy.
A "P.W. Langsdale" is recorded as at Washington Medical College in the 1830s, though I don't think this is him. I think that he graduated from Univesity of Pennsylvania Medical College in 1838. See J. Howe (Jedidiah Howe) Adams, History of the Life of D. Hayes Agnew (Philadelphia, 1892): 50-51, where he is described as a classmate of Agnew's in 1838, and as "retired Medical Director, U.S. Navy." He entered the Navy in November of 1844.
On December 21st, 1847, President Polk nominated "Philip Lansdale, Alexander J. Rice, John A. Pettit, Thomas B. Steele, James F. Harrison, and A. Nelson Bell to be assistant surgeons in the Navy from the 5th of March, 1847, at which time they were appointed according to law" (Journal of the Executive Proceedings of the Senate of the United States of America, vol. 7, page 271; see at www.memory.loc.gov).
See the image attached, a "bill for the relief of Philip Lansdale"; he had apparently been serving on the "John Adams" between April, 1859 and February, 1861. The bill was passed; note that there are two other versions of the same bill.
On Thursday, February 12, 1861, he was nominated by the President "to be a Surgeon in the Navy, from the 20th of January, 1861, vice Surgeon W. A. W. Spottswood, resigned" (Journal of the Executive Proceedings of the Senate of the United States of America, vol. 11, page 261; see at www.memory.loc.gov).
During the Civil War he appears as "Surgeon Lansdale" serving in Oct. 1863 aboard the U.S.S. Pensacola, and in 1864 serving with Farragut in the blockading squadron in Mobile Bay; he served there on the Flagship "Hartford." Capt. Percival Drayton's report of August 6, 1864, mentions after an engagement that "The devoted attention of Fleet-Surgeon Palmer, Surgeon Lansdale, and Assistant-Surgeon Commons to our wounded was beyond praise, and it was owing to their skill and untiring exertions that the large number of desperately wounded were prepared by eight o'clock in the evening for removal to the hospital at Pensacola, for which place they left at daylight on the following morning in the ‘Metacomet,' under a flag of truce" (this excerpt taken from Frank Moore, Ed., Rebellion Record: a Diary of American Events: Documents and Narratives, vol. 8, page 108, in the chapter on the "Surrender of Fort Powell."). A .pdf copy of Drayton's report from the Official Records is attached. He is included on the Civil War page.
On December 4, 1873, he was nominated by the President "to be a medical director in the Navy from the 8th of June, 1873, vice Medical Director Niman Pinckney, retired" (Journal of the Executive Proceedings of the Senate of the United States of America, vol. 21, page 162; see at www.memory.loc.gov).
Elihu Samuel Riley, in "The Ancient City.": A History of Annapolis, in Maryland, 1649-1887 (Annapolis, 1887): 217 describes him as "Medical Directory, U.S. Navy."
The Library of Congress has this item in its manuscript collection: "Autograph collection of Philip Lansdale, 1771-1816," 19 items, MSS5370. I do not know whether this is the same Philip Lansdale.