It sounds like the company is getting you to take the online tests administered by Fourth Valley Concierge (see here), specifically selecting PHP for the programming language component. You would need to speak to the company in question to see whether this is the only component you will be taking, or if they are requiring you to do the full UNITT test.
As for the test itself, the only information I have located indicates it is composed of a series of multiple-choice questions, and (for the programming component) practical programming skill questions performed through an online testing structure. From their 2012 tests:
Round 1: an examination of logical thinking, technical knowledge and business knowledge. The subject of technical knowledge includes questions from four categories: algorithms, database, security, and network. The test time is 60 minutes, and the examination includes multi-choice questions.
Round 2: an examination measuring your programming skills. You may choose one programming language among Java, PHP and C++. The examination consists of knowledge-based multi-choice questions and practical programming skill questions. The test time is 35 minutes.
As for preparation, a basic knowledge and familiarity with PHP (since this is apparently the language required for this company), as well as a firm grasp of the basic skills (covering the four main test categories of algorithms, database, security and network) would probably be useful, particularly how to implement standard algorithms, access and manipulate databases, maintain security and interface with the network in the given language. You might also want to speak to the company who is asking you to take the test - explaining that while you are familiar with PHP (which I hope you are given this is what they are testing you for), that you are not familiar with the specific test they have indicated and ask them whether they have more information on it they could give you.