Creating the actual extension source is as easy as: 

$ pecl-gen x1.xml

Creating 'x1' extension in '/home/hartmut/projects/x1'
Your extension has been created in directory x1.
See x1/README for further instructions.
This will create the following directory from x1.xml: 

x1
|-- EXPERIMENTAL
|-- README
|-- config.m4
|-- manual
|   `-- x1
|       |-- configure.xml
|       |-- functions
|       `-- reference.xml
|-- package.xml
|-- php_x1.h
|-- tests
|-- x1.c
`-- x1.dsp