Working with variables
Working with classes and objects
PHP at the Core: A Hacker's Guide to the Zend Engine
PHP Manual

# Writing functions

PHP is also known as a Glue language, and extending it, can be easily done with those extensions generators. When you use ext_skel and a prototype file to generate the C function stubs, you will notice that all of the exported functions created have a simple prototype such as the following: PHP_FUNCTION(func_name)

Working with variables
Working with classes and objects
PHP at the Core: A Hacker's Guide to the Zend Engine
PHP Manual