I have the follow problem.
First I installed udc_snmp 4.2.1 ( I tryed also 4.1.2).

then T treyed to compile PHP4.05 ( also 4.06)

./configure --with-snmp=/usr/local
make

 I get follow error:
bin/sh /export/home/php-4.0.5/libtool --silent --mode=link gcc  -I. -I/export/home/php-4.0.5/ -I/export/home/php-4.0.5/main -I/export/home/php-4.0.5 -I/export/home/php-4.0.5/Zend -I/export/home/php-4.0.5/ext/mysql/libmysql -I/usr/local/include/ucd-snmp -I/export/home/php-4.0.5/ext/xml/expat/xmltok -I/export/home/php-4.0.5/ext/xml/expat/xmlparse -I/export/home/php-4.0.5/TSRM  -D_POSIX_PTHREAD_SEMANTICS -DSUPPORT_UTF8 -DXML_BYTE_ORDER=21 -g -O2   -o php -export-dynamic stub.lo libphp4.la 
Undefiniertes                   erstmals referenziert
 Symbol                             in Datei
kstat_close                         /usr/local/lib/libsnmp.so
kstat_lookup                        /usr/local/lib/libsnmp.so
kstat_read                          /usr/local/lib/libsnmp.so
kstat_open                          /usr/local/lib/libsnmp.so
kstat_data_lookup                   /usr/local/lib/libsnmp.so
ld: Schwerer Fehler: Symbolreferenzierungsfehler. Keine Ausgabe in php geschrieben
collect2: ld returned 1 exit status
*** Error code 1
make: Fatal error: Command failed for target `php'
Current working directory /export/home/php-4.0.5
*** Error code 1
make: Fatal error: Command failed for target `all-recursive'

Anywhere I reed to use the -lkstat option, 
I tryed last command with -lkstat again

bin/sh /export/home/php-4.0.5/libtool --silent --mode=link gcc  -I. -I/export/home/php-4.0.5/   -I/export/home/php-4.0.5/main -I/export/home/php-4.0.5 -I/export/home/php-4.0.5/Zend -I/export/home/php-4.0.5/ext/mysql/libmysql -I/usr/local/include/ucd-snmp -I/export/home/php-4.0.5/ext/xml/expat/xmltok -I/export/home/php-4.0.5/ext/xml/expat/xmlparse -I/export/home/php-4.0.5/TSRM  -D_POSIX_PTHREAD_SEMANTICS -DSUPPORT_UTF8 -DXML_BYTE_ORDER=21 -g -O2   -o php -export-dynamic stub.lo libphp4.la -lkstat

Then I did 
make 
make install

When I start php now, I get a core dump.
I don't have this problem without the snmp module.

Best regards
S. Schmidt