Green’s Dictionary of Slang

(UK Und.) an excl. that indicates the approach of the police.

1873		Sl. Dict. 251: Philip a policeman. The word is loudly given as a signal that the police are approaching.
1880		W. Hooe Sharping London 35: Philip, a word used to warn the approach of police.
1886		Graphic (London) 30 Jan. 23/1: Another name is ‘Philip,’ this being the signal word loudly given by a thief’s accomplice to announce the near approach of the police.
1895		C. Crowe Aus. Sl. Dict. 57: Philip, a thief’s signal, signifying that the police are coming.