SNMP::__construct() fails to resolve hostname with warning "Unknown failure while resolving hostname"

Description

The following code:

<?php

$session = new SNMP(SNMP::VERSION_2C, '127.0.0.1', 'public');
var_dump($session);

Resulted in this output:

PHP Warning:  SNMP::__construct(): Unknown failure while resolving '127.0.0.1' in /tmp/snmp_create_session.php on line 3
object(SNMP)#1 (8) {
  ["info"]=>
  array(3) {
    ["hostname"]=>
    string(0) ""
    ["timeout"]=>
    int(-1)
    ["retries"]=>
    int(-1)
  }
  ["max_oids"]=>
  NULL
  ["valueretrieval"]=>
  int(0)
  ["quick_print"]=>
  bool(false)
  ["enum_print"]=>
  bool(false)
  ["oid_output_format"]=>
  int(0)
  ["oid_increasing_check"]=>
  bool(false)
  ["exceptions_enabled"]=>
  int(0)
}

But I expected this output instead:

object(SNMP)#1 (8) {
  ["info"]=>
  array(3) {
    ["hostname"]=>
    string(9) "127.0.0.1"
    ["timeout"]=>
    int(-1)
    ["retries"]=>
    int(-1)
  }
  ["max_oids"]=>
  NULL
  ["valueretrieval"]=>
  int(0)
  ["quick_print"]=>
  bool(false)
  ["enum_print"]=>
  bool(false)
  ["oid_output_format"]=>
  int(0)
  ["oid_increasing_check"]=>
  bool(true)
  ["exceptions_enabled"]=>
  int(0)
}

The problem reproduces on 8.5.1, 8.4.16, and 8.4.15.

PHP Version

PHP 8.4.15 (cli) (built: Dec 11 2025 18:42:43) (NTS)
Copyright (c) The PHP Group
Zend Engine v4.4.15, Copyright (c) Zend Technologies
    with Zend OPcache v8.4.15, Copyright (c), by Zend Technologies

PHP 8.5.1 (cli) (built: Dec 21 2025 12:04:55) (NTS)
Copyright (c) The PHP Group
Zend Engine v4.5.1, Copyright (c) Zend Technologies
    with Zend OPcache v8.5.1, Copyright (c), by Zend Technologies

Operating System

Ubuntu 24.04.3 LTS