I've got the fix building on PHP-8 branch.  It's to add a new function:

snmp_string_output_format(blah);

Do you want a patch, or a pull request?  If so, from what branch?

As this is a new feature this can only be added in PHP 8.1.0.

Please provide a PR to the master branch of php-src on GitHub.
It might also need an RFC depending on the scope.

For some reason on the master branch and stock source, when I run ./configure --with-snmp, I'm getting errors.  I was able to compile just fine on PHP-7.3.26 with my changes though.

I had had to slightly modify the source for 7.3.26 due to some changes, making changes to php-snmp with the stock master as the basis, with --with-snmp is failing.  My guess is that there have been a bunch of type definition changes that have not been migrated to 'master'.  There's been a lot of changes between releases, so I'll track the commits and wait for some upstream fixes before doing a pull.

Here is what I get with stock master:

[root@vmhost3 php-src-master]# make
/bin/sh /root/php-src-master/libtool --silent --preserve-dup-deps --mode=compile cc -std=gnu99 -Iext/snmp/ -I/root/php-src-master/ext/snmp/ -I/root/php-src-master/include -I/root/php-src-master/main -I/root/php-src-master -I/root/php-src-master/ext/date/lib -I/usr/include/libxml2 -I/root/php-src-master/TSRM -I/root/php-src-master/Zend    -fno-common -Wlogical-op -Wno-clobbered -Wall -Wextra -Wno-strict-aliasing -Wno-implicit-fallthrough -Wno-unused-parameter -Wno-sign-compare -g -O2 -fvisibility=hidden -DZEND_SIGNALS    -c /root/php-src-master/ext/snmp/snmp.c -o ext/snmp/snmp.lo
In file included from /root/php-src-master/main/php.h:35:0,
                 from /root/php-src-master/ext/snmp/snmp.c:26:
/root/php-src-master/Zend/zend_API.h:77:88: error: expected expression before ‘,’ token
 #define ZEND_RAW_FENTRY(zend_name, name, arg_info, flags)   { zend_name, name, arg_info, (uint32_t) (sizeof(arg_info)/sizeof(struct _zend_internal_arg_info)-1), flags },
                                                                                        ^
/root/php-src-master/Zend/zend_API.h:83:38: note: in expansion of macro ‘ZEND_RAW_FENTRY’
 #define ZEND_FE(name, arg_info)      ZEND_RAW_FENTRY(#name, zif_##name, arg_info, 0)
                                      ^
/root/php-src-master/ext/snmp/snmp_arginfo.h:217:2: note: in expansion of macro ‘ZEND_FE’
  ZEND_FE(snmp_set_string_output_format, arginfo_snmp_set_string_output_format)
  ^
/root/php-src-master/Zend/zend_API.h:77:88: warning: missing initializer for field ‘arg_info’ of ‘zend_function_entry’ [-Wmissing-field-initializers]
 #define ZEND_RAW_FENTRY(zend_name, name, arg_info, flags)   { zend_name, name, arg_info, (uint32_t) (sizeof(arg_info)/sizeof(struct _zend_internal_arg_info)-1), flags },
                                                                                        ^
/root/php-src-master/Zend/zend_API.h:83:38: note: in expansion of macro ‘ZEND_RAW_FENTRY’
 #define ZEND_FE(name, arg_info)      ZEND_RAW_FENTRY(#name, zif_##name, arg_info, 0)
                                      ^
/root/php-src-master/ext/snmp/snmp_arginfo.h:217:2: note: in expansion of macro ‘ZEND_FE’
  ZEND_FE(snmp_set_string_output_format, arginfo_snmp_set_string_output_format)
  ^
In file included from /root/php-src-master/main/php.h:35:0,
                 from /root/php-src-master/ext/snmp/snmp.c:26:
/root/php-src-master/Zend/zend_API.h:38:40: note: ‘arg_info’ declared here
  const struct _zend_internal_arg_info *arg_info;
                                        ^
cc1: warning: unrecognized command line option "-Wno-implicit-fallthrough" [enabled by default]
make: *** [ext/snmp/snmp.lo] Error 1

Okay, getting a hang of the code now.

Pull request is here.  Passed testing it looks.

https://github.com/php/php-src/pull/6633