It's just Hinduism with a mixture of South-East Asian animism. It's quite similar to Hinduism in Bali and Eastern Java. The dominant sect in South-East Asian Hinduism was always Shaivite. Some elements of SE Hinduism crept into Theravada Buddhism practiced there, such as veneration of Brahma.
http://en.wikipedia.org/wiki/Phra_PhromWorship of Brahma is very rare in Hinduism. There were only two major temples in history - one in Pushkar in India (still of importance), the other in the Khmer Empire. Hence the veneration of Phra Phrom.