Doi Pha Hom Pok NP is located in the far north-west of Thailand next to the border with Myanmar. There are two separate entrance roads - the 'west' access road starts from Fang and the 'east' road starts from Thaton. Most birders refer to the whole site as 'Doi Lang' as this is the name of the mountain accessible by the forementioned roads.
Birding is mainly done along the roads as there are few trails, but the area has very little traffic and the roadside forest is often teeming with birdlife. The western side is drier and consists mainly of pine forest at the higher elevations, with some pockets of evergreen forest. The eastern side receives more rainfall and evergreen forest dominates. At lower and mid elevations the forest can be quite patchy and there are a few scattered farms which seem to have been abandoned. Higher on the mountain there are decent areas of primary forest.
A 4wd or high-clearance pickup is necessary for the eastern road, and apparently they won't let you through the first military checkpoint without one. On both sides, military checkpoints block the road at the highest points, and the border police we dealt with were super-friendly and happy for us to park at the checkpoint and walk further up the road.
Birding is excellent - this is perhaps the best site in Thailand to see a wide range of birds from the 'Himalayan' bioregion. Species such as Mountain Bamboo Partridge, Hume's Pheasant, Giant Nuthatch, Spot-breasted and Grey-headed Parrotbill, Red-tailed, White-browed, Silver-eared & Spot-breasted Laughingthrush, Red-faced Liocichla, Himalayan Cutia, Whiskered Yuhina and Humes' Treecreeper are all regulars. In winter, migratory thrushes, warblers and robins boost the bird list further, as Doi Lang is a relatively mild retreat for many species escaping the icy weather farther north in China and Myanmar.
Good maps and more detailed information can be found here -
www.norththailandbirding.com/pages/localities/doi_lang.html
During my visit in early March 2016, the birdlife was prolific, although mammals seemed a bit thin on the ground. A few different squirrels and a treeshrew were seen, and barking deer and a langur species were heard. Yellow-throated Marten are supposed to be common but we didn't see any.
Here's a few photos from the trip...