PHP gethostbyname Function

last modified April 4, 2025

The PHP gethostbyname function resolves hostnames to IP addresses. It performs forward DNS lookups, converting domain names to IPv4 addresses.

Basic Definition

gethostbyname translates a hostname to its corresponding IPv4 address. It queries DNS servers to perform the resolution. Returns a string with the IP.

Syntax: gethostbyname(string $hostname): string. Returns the IP address or the original hostname if resolution fails. Simple but effective.

Basic Hostname Resolution

This example demonstrates the simplest use of gethostbyname.

basic_resolution.php

<?php

declare(strict_types=1);

$hostname = "www.example.com";
$ip = gethostbyname($hostname);

echo "The IP address of $hostname is: $ip";

This code resolves www.example.com to its IPv4 address. If resolution fails, it returns the original hostname. Always verify the result is an IP.

Checking Resolution Success

This example shows how to verify if hostname resolution was successful.

check_resolution.php

<?php

declare(strict_types=1);

$hostname = "www.example.com";
$ip = gethostbyname($hostname);

if ($ip !== $hostname) {
    echo "Resolution successful: $ip";
} else {
    echo "Resolution failed for $hostname";
}

The function returns the input hostname if resolution fails. This pattern helps distinguish between success and failure cases clearly.

Resolving Multiple Hostnames

This example resolves multiple hostnames in one operation efficiently.

multi_resolution.php

<?php

declare(strict_types=1);

$hostnames = ["google.com", "yahoo.com", "github.com"];

foreach ($hostnames as $host) {
    $ip = gethostbyname($host);
    echo "$host resolves to $ip\n";
}

This demonstrates batch processing of hostname resolution. Useful for applications needing to resolve multiple domains at once.

Building a URL Validator

This example creates a URL validator that checks domain resolution.

url_validator.php

<?php

declare(strict_types=1);

function validateUrl($url) {
    $parsed = parse_url($url);
    if (!isset($parsed['host'])) return false;
    
    $ip = gethostbyname($parsed['host']);
    return $ip !== $parsed['host'];
}

$url = "https://www.example.com";
echo validateUrl($url) ? "Valid URL" : "Invalid URL";

This function extracts the hostname from a URL and verifies it resolves. Combine with other validation for comprehensive URL checking.

Comparing with gethostbynamel

This example shows the difference between gethostbyname and gethostbynamel.

compare_resolution.php

<?php

declare(strict_types=1);

$hostname = "www.google.com";

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

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

gethostbyname returns one IP while gethostbynamel returns all. Useful when hosts have multiple IP addresses for load balancing.

Best Practices

Error Handling: Always check if resolution succeeded
Caching: Cache results to avoid repeated DNS queries
IPv6: Use dns_get_record for IPv6 support
Timeouts: Consider DNS query timeout in production

Source

PHP gethostbyname Documentation

This tutorial covered the PHP gethostbyname function with practical examples for hostname to IP resolution in various scenarios.

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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