Basically a duplicate of #7843 with detailed infos and fixes:

Required source patches (beware of the wraps):
===========================================================
--- cmp/php-4.0.3pl1/ext/dav/dav.c      Sat Jun 24 17:31:06 2000
+++ src/php-4.0.3pl1/ext/dav/dav.c      Sun Nov 12 19:10:00 2000
@@ -150,7 +150,7 @@
 }
 
 
-PHP_MINFO_FUNCTION(phpdav);
+PHP_MINFO_FUNCTION(phpdav)
 {
        php_info_print_table_start();
        php_info_print_table_row(2, "DAV Support", "enabled");
--- cmp/php-4.0.3pl1/ext/dav/php_dav.h  Mon Jul 24 03:39:43 2000
+++ src/php-4.0.3pl1/ext/dav/php_dav.h  Sun Nov 12 19:07:11 2000
@@ -23,6 +23,8 @@
 
 #if HAVE_MOD_DAV
 
+#include <httpd.h>
+
 typedef struct {
     int foo;
     char *mkcol_test_handler;
diff -ru cmp/php-4.0.3pl1/sapi/apache/mod_php4.c src/php-4.0.3pl1/sapi/apache/mod_php4.c
--- cmp/php-4.0.3pl1/sapi/apache/mod_php4.c     Sat Oct 14 22:06:20 2000
+++ src/php-4.0.3pl1/sapi/apache/mod_php4.c     Sun Nov 12 20:41:54 2000
@@ -743,7 +743,7 @@
 
        conf = (php_apache_info_struct *) get_module_config(r->per_dir_config,
 
                        &php4_module);
-       return send_php(r, 0, 0, conf->dav_script);
+       return send_php(r, 0, conf->dav_script);
 }
 
 static int php_type_checker(request_rec *r)
===========================================================

Other required steps:
Copy mod_dav.h from the mod_dav source directory to the apache include directory (usually /usr/local/apache/include), then use the following switch for php configuration:
--with-mod-dav=/usr/local/apache/include

Note:
Only build test was done, no functional test!