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copied!<p>First, validate that it is a valid URL using <a href="http://php.net/filter_var" rel="nofollow noreferrer"><code>filter_var()</code></a>:</p> <pre><code><?php $url = "http://www.example.com/foo.php"; if(!filter_var($url, FILTER_VALIDATE_URL)) { die('Invalid URL'); } </code></pre> <p>Next, parse the URL with <a href="http://php.net/parse_url" rel="nofollow noreferrer"><code>parse_url()</code></a> and ensure that it is HTTP(S):</p> <pre><code>$p_url = parse_url($url); if(!$p_url) // couldn't parse URL, since parse_url() cannot recognize it { die('Invalid URL'); } if($p_url['scheme'] != 'http' && $p_url['scheme'] != 'https') { die('Invalid protocol (only HTTP(S) is supported)'); } </code></pre> <p>Lastly, check that the host exists and that you can connect to it. I choose to use <a href="http://php.net/fsockopen" rel="nofollow noreferrer"><code>fsockopen()</code></a> here, since it would check the hostname and port, while not actually sending an HTTP request.</p> <pre><code>$fp = fsockopen($p_url['host'], (isset($p_url['port']) ? $p_url['port'] : 80)); if($fp) { echo 'Valid URL'; fclose($fp); // Remember to close the file pointer }else{ echo 'Invalid server'; } </code></pre> <p>Note that you might want to avoid using this method (depending on what you want your application to do), as if the server is down, this would result in <code>Invalid server</code>, even though the server might exist. An alternative solution that will only check the hostname from the DNS servers, and not connect at all, is using <a href="http://php.net/gethostbyaddr" rel="nofollow noreferrer"><code>gethostbyaddr()</code></a> and <a href="http://php.net/gethostbyname" rel="nofollow noreferrer"><code>gethostbyname()</code></a>:</p> <pre><code>if(@gethostbyaddr($p_url['host'])) // IP addresses passes this test { echo 'Valid URL'; }else{ $host = $p_url['host']; $addr = gethostbyname($host); if(!$addr) // Invalid domain name { echo 'Invalid domain name'; }else if($host == $addr) // Domain name could not be resolved (i.e. does not exist) { echo 'Invalid domain name'; }else{ echo 'Valid URL'; } } </code></pre> <p>Links:</p> <ul> <li><a href="http://php.net/filter_var" rel="nofollow noreferrer"><code>filter_var()</code></a> - Validate strings</li> <li><a href="http://php.net/parse_url" rel="nofollow noreferrer"><code>parse_url()</code></a> - Parses URLs into arrays</li> <li><a href="http://php.net/fsockopen" rel="nofollow noreferrer"><code>fsockopen()</code></a> - Opens a socket connection</li> <li><a href="http://php.net/gethostbyaddr" rel="nofollow noreferrer"><code>gethostbyaddr()</code></a> - Gets the domain name of an IP address (not sure if it supports IPv6, though)</li> <li><a href="http://php.net/gethostbyname" rel="nofollow noreferrer"><code>gethostbyname()</code></a> - Gets the IPv4 address of a domain name</li> </ul>

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