The documentation for extensions is located in:

/phpdoc/en/trunk/reference/extension_name/

For example, the SVN extension documentation exists in:

/phpdoc/en/trunk/reference/svn/

There you'll find several files:

• book.xml (Acts as the container for the extension and contains the preface. Other files (like examples.xml are included from here)
• setup.xml (Includes setup, install and configuration documentaiton)
• constants.xml (Lists all constants the extension declares, if any)
• configure.xml (Usually this information is in setup.xml, but if the file exists it is magically included into setup.xml)
• examples.xml (Various examples)
• foo.xml (Example, foo can be anything specific to a topic. Just be sure to include via book.xml)

A procedural extension (like SVN) also has:

• reference.xml (container for the functions, rarely contains any info)
• functions/ (folder with one XML file per function that the extension declares)

And OO extensions (such as imagick) contain:

• classname.xml (container for the methods defined by the class, contains also basic info about it)
• classname/ (folder with one XML per method that the class declares)

• Note: ”classname” is the lowercased name of the class, not a literal file or directory name.