Pre-Angkor period, Sambor-Prei-Kuk style, 7th-8th century.
There's always a lingam in the cella (heart) of the god Shiva's temples. The lingam is the representation of Shiva in his 'absolute' form. The anthropomorphic images of Shiva only show certain aspects of the god. Both gods Shiva and Visnu (his father) have the ability to present themselves to their devotees in a wide range of different manifestations, depending on which aspect of their all-encompassing nature is appropriate for the specific location, situation and need of the believer.
As opposite to this philosophical meaning, the lingam is also seen as the phallus of the God Shiva, and as the sign mark for his power to create.
This specific form, which rejuvenates downwards, is characteristic for the Sambor-Prei-Kruk style of the Pre-Angkor period. This form dissappears in later periods.