AMQPHP is an Amqp implementation written in pure PHP (the clue's in the name!). AMQPHP is written using namespaces (PHP 5.3+) and features an Phing/XSLT based code generation system. Using code generation means that AMQPHP can more easily keep up with changes in the Amqp 0.9.1 protocol, for example exchange to exchange bindings in RabbitMQ can be supported without re-writing any PHP code. AMQPHP can be used to write both Amqp producers and consumers, and can use SSL, with client certificates and peer-verification.
Amqphp must be built using the standard PHP build tool - Phing. If you don't want to use Phing, download one of the tagged releases from Github, these always include pre-built sources. There are 2 varieties of build - CPF i.e. Class Per File, and NSPF i.e. Namespace per file. Use the former if you need to autoload classes, use the latter if you don't.
This manual covers all aspects of using the Amqphp API, but it does not cover and broker architecture, configuration or any other general Amqp topics. There are many places on the web which cover this material, I'd recommend the RabbitMQ concepts tutorial as a good place to start.