Run the ext_skel script which creates all the required skeleton files. 

% cd php4/ext
% ./ext_skel --extname=tidy --proto=tidy.def
Creating directory tidy
Creating basic files: config.m4 Makefile.in .cvsignore tidy.c 
                      php_tidy.h CREDITS EXPERIMENTAL 
                      tests/001.phpt tidy.php [done].
% ls tidy
config.m4    CREDITS       EXPERIMENTAL    tidy.c 
tidy.php     Makefile.in   php_tidy.h      tests/
Standalone or Embedded?
At this point you have a choice on how to proceed. You can either develop your extension under the main PHP framework where you run the top-level buildconf script to include your extension in PHP's main configuration script, or you can develop your extension in a completely separate directory where you run the phpize script to populate your extension directory with all the required files.

Embedded
% cd tidy
% vi config.m4
% cd ../..
% ./buildconf
% ./configure --[with|enable]-tidy
% make
Standalone
% mv tidy /tmp
% cd /tmp/tidy
% vi config.m4
% phpize
% ./configure --[with|enable]-tidy
% make