PHP gethostbynamel Function

last modified April 4, 2025

The PHP gethostbynamel function returns all IP addresses associated with a hostname. It's useful when a domain has multiple IPs.

Basic Definition

gethostbynamel resolves a hostname to an array of IPv4 addresses. It performs a DNS A record lookup for the given domain name.

Syntax: gethostbynamel(string $hostname): array|false. Returns an array of IPs or false on failure. Requires DNS server access.

Basic Hostname Resolution

This example shows how to get all IP addresses for a hostname.

basic_resolution.php

<?php

declare(strict_types=1);

$hostname = "www.google.com";
$ips = gethostbynamel($hostname);

if ($ips !== false) {
    echo "IP addresses for $hostname:\n";
    print_r($ips);
} else {
    echo "Could not resolve $hostname";
}

This code resolves www.google.com to its IP addresses. Large sites often have multiple IPs for load balancing and redundancy.

Handling Resolution Failure

This demonstrates proper error handling when hostname resolution fails.

error_handling.php

<?php

declare(strict_types=1);

$hostname = "nonexistent.example";
$ips = gethostbynamel($hostname);

if ($ips === false) {
    echo "Failed to resolve $hostname\n";
    $error = error_get_last();
    echo "Error: " . $error['message'];
}

When resolution fails, the function returns false. We check for this and display the error message from PHP's error system.

Checking Localhost Resolution

This example checks how localhost resolves to IP addresses.

localhost_check.php

<?php

declare(strict_types=1);

$hostname = "localhost";
$ips = gethostbynamel($hostname);

if ($ips !== false) {
    echo "localhost resolves to:\n";
    foreach ($ips as $ip) {
        echo "- $ip\n";
    }
} else {
    echo "Failed to resolve localhost";
}

Localhost should typically resolve to 127.0.0.1. This verifies your system's hostname resolution is working correctly.

Comparing with gethostbyname

This shows the difference between gethostbynamel and gethostbyname.

comparison.php

<?php

declare(strict_types=1);

$hostname = "www.example.com";

$single_ip = gethostbyname($hostname);
$all_ips = gethostbynamel($hostname);

echo "Single IP: $single_ip\n";
echo "All IPs: " . implode(', ', $all_ips);

gethostbyname returns just one IP, while gethostbynamel returns all. This is important for services with multiple servers.

Building a Host Checker Tool

This example creates a complete host checker using gethostbynamel.

host_checker.php

<?php

declare(strict_types=1);

function checkHost($hostname) {
    $ips = gethostbynamel($hostname);
    
    if ($ips === false) {
        return "Host $hostname could not be resolved";
    }
    
    $result = "Host $hostname resolves to:\n";
    foreach ($ips as $ip) {
        $result .= "- $ip\n";
        $result .= "Reverse: " . gethostbyaddr($ip) . "\n";
    }
    
    return $result;
}

echo checkHost("www.php.net");

This tool resolves a hostname and shows all IPs with reverse DNS lookups. It demonstrates practical use of gethostbynamel in a complete function.

Best Practices

Caching: Cache DNS results to reduce lookup times
Timeouts: Consider DNS query timeout settings
Validation: Validate hostnames before resolution
Error Handling: Always check for false return value
IPv6: Use dns_get_record for IPv6 support

Source

PHP gethostbynamel Documentation

This tutorial covered the PHP gethostbynamel function with practical examples for resolving hostnames to multiple IP addresses.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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