The actual Windows path may be C:\usr\share\snmp\mibs
E.g. 'C:\' concatenated with $mibdir (which is '/usr/share/snmp/mibs')
https://github.com/php/php-src/blob/75ed2091bca359f5bb1f1819a03f04c57be82b84/ext/snmp/tests/snmp_include.inc#L23
At least this is where PhpStan looks for it as well.