PHP4 & Apache 1.3.9 Install

I am trying to install php4 as a module into Apache. However, the php4
modules make keeps trying to install Zend files (which I'm clueless about as
far as what Zend is). Outside of the php4 source, Apache compiles fine; it
even configures with the php4 source. The only thing keeping it from
compiling with the php4 source is a bunch of errors saying it cant find the
following files:

zend.h
php.h
php_variables.h
SAPI.h
php_ini.h
php_globals.h
SAPI.h
main.h
zend_compile.h
zend_execute.h
zend_highlight.h
zend_indent.h
ext/standard/php_standard.h


Help would be greatly appreciated.

Ben