On a nearly blank, vanilla-flavored Intel installation of FreeBSD 4.6-RELEASE with apache-1.3.24_7 and mysql-3.23.49 (both built from the ports tree), a pathing issue exists with the *.h files in net-snmp-5.0.3 when building php-4.2.2 configured --with-snmp=/usr/local:

Locally (not package installs, or builds from the ports tree), builds and installs of  zlib-1.1.4 (./configure --prefix=/usr/local --libdir=/usr/local/lib --includedir=/usr/local/include), openssl-0.9.6e (with no options to configure), and then net-snmp-5.0.3 (./configure --with-openssl=/usr/local --with-mib-modules="host misc/ipfwacc disman/event-mib" --enable-embedded-perl).  After installation of net-snmp, built gd-1.8.4, then started build of php-4.2.2 (./configure --with-apxs --with-openssl=/usr/local/ssl --with-mysql=/usr/local --with-gd=/usr/local --with-snmp=/usr/local --enable-ucd-snmp-hack).  

Build failed when attempting to compile ext/snmp.c

Below are the unified diffs of a freshly untar'd copy of php-4.2.2, and the modified version that built for me.

diff -ru php-4.2.2/configure php-4.2.2p1/configure
--- php-4.2.2/configure Sun Jul 21 05:56:19 2002
+++ php-4.2.2p1/configure       Sat Aug  3 14:01:15 2002
@@ -57912,16 +57912,16 @@
     if test "$PHP_SNMP" = "yes"; then
       for i in /usr/include /usr/local/include; do
         test -f $i/snmp.h                       && SNMP_INCDIR=$i
-        test -f $i/ucd-snmp/snmp.h              && SNMP_INCDIR=$i/ucd-snmp
+        test -f $i/net-snmp/snmp.h              && SNMP_INCDIR=$i/net-snmp
         test -f $i/snmp/snmp.h                  && SNMP_INCDIR=$i/snmp
-        test -f $i/snmp/include/ucd-snmp/snmp.h && SNMP_INCDIR=$i/snmp/include/ucd-snmp
+        test -f $i/snmp/include/net-snmp/snmp.h && SNMP_INCDIR=$i/snmp/include/net-snmp
       done
       for i in /usr /usr/snmp /usr/local /usr/local/snmp; do
         test -f $i/lib/libsnmp.a -o -f $i/lib/libsnmp.$SHLIB_SUFFIX_NAME && SNMP_LIBDIR=$i/lib
       done
     else
       SNMP_INCDIR=$PHP_SNMP/include
-      test -d $PHP_SNMP/include/ucd-snmp && SNMP_INCDIR=$PHP_SNMP/include/ucd-snmp
+      test -d $PHP_SNMP/include/net-snmp && SNMP_INCDIR=$PHP_SNMP/include/net-snmp
       SNMP_LIBDIR=$PHP_SNMP/lib
     fi
 
@@ -57980,7 +57980,7 @@
 #line 57981 "configure"
 #include "confdefs.h"
 
-        #include <ucd-snmp-config.h>
+        #include <net-snmp-config.h>
         #if USE_OPENSSL
         yes
         #endif

diff -ru php-4.2.2/ext/snmp/config.m4 php-4.2.2p1/ext/snmp/config.m4
--- php-4.2.2/ext/snmp/config.m4        Wed Jan 16 14:32:40 2002
+++ php-4.2.2p1/ext/snmp/config.m4      Sat Aug  3 14:02:30 2002
@@ -10,16 +10,16 @@
     if test "$PHP_SNMP" = "yes"; then
       for i in /usr/include /usr/local/include; do
         test -f $i/snmp.h                       && SNMP_INCDIR=$i
-        test -f $i/ucd-snmp/snmp.h              && SNMP_INCDIR=$i/ucd-snmp
+        test -f $i/net-snmp/snmp.h              && SNMP_INCDIR=$i/net-snmp
         test -f $i/snmp/snmp.h                  && SNMP_INCDIR=$i/snmp
-        test -f $i/snmp/include/ucd-snmp/snmp.h && SNMP_INCDIR=$i/snmp/include/ucd-snmp
+        test -f $i/snmp/include/net-snmp/snmp.h && SNMP_INCDIR=$i/snmp/include/net-snmp
       done
       for i in /usr /usr/snmp /usr/local /usr/local/snmp; do
         test -f $i/lib/libsnmp.a -o -f $i/lib/libsnmp.$SHLIB_SUFFIX_NAME && SNMP_LIBDIR=$i/lib
       done
     else
       SNMP_INCDIR=$PHP_SNMP/include
-      test -d $PHP_SNMP/include/ucd-snmp && SNMP_INCDIR=$PHP_SNMP/include/ucd-snmp
+      test -d $PHP_SNMP/include/net-snmp && SNMP_INCDIR=$PHP_SNMP/include/net-snmp
       SNMP_LIBDIR=$PHP_SNMP/lib
     fi
 
@@ -35,7 +35,7 @@
     if test "$ac_cv_header_default_store_h" = "yes"; then
       AC_MSG_CHECKING(for OpenSSL support in SNMP libraries)
       AC_EGREP_CPP(yes,[
-        #include <ucd-snmp-config.h>
+        #include <net-snmp-config.h>
         #if USE_OPENSSL
         yes
         #endif

diff -ru php-4.2.2/ext/snmp/snmp.c php-4.2.2p1/ext/snmp/snmp.c
--- php-4.2.2/ext/snmp/snmp.c   Thu Feb 28 19:31:01 2002
+++ php-4.2.2p1/ext/snmp/snmp.c Sat Aug  3 14:26:18 2002
@@ -59,13 +59,14 @@
 #ifdef HAVE_DEFAULT_STORE_H
 #include "default_store.h"
 #endif
-#include "asn1.h"
-#include "snmp_api.h"
-#include "snmp_client.h"
-#include "snmp_impl.h"
-#include "snmp.h"
-#include "parse.h"
-#include "mib.h"
+#include "net-snmp-config.h"
+#include "library/asn1.h"
+#include "library/snmp_api.h"
+#include "library/snmp_client.h"
+#include "library/snmp_impl.h"
+#include "library/snmp.h"
+#include "library/parse.h"
+#include "library/mib.h"
 #include "version.h"
 
 /* ucd-snmp 3.3.1 changed the name of a few #defines... They've been changed back to the original ones in 3.5.3! */
@@ -130,7 +131,7 @@
 {
        php_info_print_table_start();
        php_info_print_table_row(2, "UCD-SNMP Support", "enabled");
-       php_info_print_table_row(2, "UCD-SNMP Version", VersionInfo);
+       php_info_print_table_row(2, "UCD-SNMP Version", NetSnmpVersionInfo);
        php_info_print_table_end();
 }
 /* }}} */

I made no changes other than those which would complete the build (no cosmetic changes of 'ucd' to 'net').  I don't know the stability of this build as I haven't stress-tested it yet, but wanted to pass this along ASAP.