The Philharmonia Orchestra was founded in 1945 by Walter Legge and is based in London. It was known as New Philharmonia Orchestra between 1964-1976, and returned to the name Philharmonia Orchestra in 1976 (see http://www.philharmonia.co.uk/about/history/).
It is not the non-professional New Philharmonia Orchestra of Massachusetts, founded in 1995 and based in Massachusetts, USA, or the London Philharmonic Orchestra, founded in 1932 by Sir Thomas Beecham, or the New Philharmonic Orchestra, a name used on budget recordings for an unknown orchestra.
However you may see it listed on older US releases as the Philharmonia Orchestra of London or the London Philharmonia Orchestra. Usually the associated conductor will be a good clue as to whether you have the correct orchestra.