It is in the tarball. You can download it from the official website
Take a look into it:
$ tar jtf php-5.4.13.tar.bz2 | grep snmp
php-5.4.13/ext/snmp/
php-5.4.13/ext/snmp/config.m4
php-5.4.13/ext/snmp/config.w32
php-5.4.13/ext/snmp/CREDITS
php-5.4.13/ext/snmp/php_snmp.h
php-5.4.13/ext/snmp/snmp.c
php-5.4.13/ext/snmp/snmp.dsp
php-5.4.13/ext/snmp/tests/
php-5.4.13/ext/snmp/tests/bug60749.phpt
php-5.4.13/ext/snmp/tests/bug64124.phpt
php-5.4.13/ext/snmp/tests/clean.inc
php-5.4.13/ext/snmp/tests/generic_timeout_error.phpt
php-5.4.13/ext/snmp/tests/ipv6.phpt
php-5.4.13/ext/snmp/tests/README
php-5.4.13/ext/snmp/tests/skipif.inc
php-5.4.13/ext/snmp/tests/snmp-object-errno-errstr.phpt
php-5.4.13/ext/snmp/tests/snmp-object-error.phpt
php-5.4.13/ext/snmp/tests/snmp-object-properties.phpt
php-5.4.13/ext/snmp/tests/snmp-object-setSecurity_error.phpt
php-5.4.13/ext/snmp/tests/snmp-object.phpt
php-5.4.13/ext/snmp/tests/snmp2_get.phpt
php-5.4.13/ext/snmp/tests/snmp2_getnext.phpt
php-5.4.13/ext/snmp/tests/snmp2_real_walk.phpt
php-5.4.13/ext/snmp/tests/snmp2_set-nomib.phpt
php-5.4.13/ext/snmp/tests/snmp2_set.phpt
php-5.4.13/ext/snmp/tests/snmp2_walk.phpt
php-5.4.13/ext/snmp/tests/snmp3-error.phpt
php-5.4.13/ext/snmp/tests/snmp3.phpt
php-5.4.13/ext/snmp/tests/snmp_get_quick_print.phpt
php-5.4.13/ext/snmp/tests/snmp_get_valueretrieval.phpt
php-5.4.13/ext/snmp/tests/snmp_getvalue.phpt
php-5.4.13/ext/snmp/tests/snmp_include.inc
php-5.4.13/ext/snmp/tests/snmp_read_mib.phpt
php-5.4.13/ext/snmp/tests/snmp_set_enum_print.phpt
php-5.4.13/ext/snmp/tests/snmp_set_oid_output_format.phpt
php-5.4.13/ext/snmp/tests/snmpd.conf
php-5.4.13/ext/snmp/tests/snmpget.phpt
php-5.4.13/ext/snmp/tests/snmpgetnext.phpt
php-5.4.13/ext/snmp/tests/snmprealwalk.phpt
php-5.4.13/ext/snmp/tests/snmpset-nomib.phpt
php-5.4.13/ext/snmp/tests/snmpset.phpt
php-5.4.13/ext/snmp/tests/snmpwalk.phpt
php-5.4.13/ext/snmp/tests/wrong_hostname.phpt