When can an alibi be considered as defense?

It is axiomatic that alibi is an inherently weak defense (People v. Arnistoso, G.R. No. 201447, January 9, 2013, 688 SCRA 376, 394, citing People v. Abulon, 557 Phil. 428, 447 [2007]), and may only be considered if the following circumstances are shown: 

(a) He was somewhere else when the crime occurred; and 
(b) It would be physically impossible for him to be at the locus criminis at the time of the alleged crime. (People v. Agcanas, G.R. No. 174476, October 11, 2011)