PHP determines whether a web link exists

To check whether a link exists, you can use the `curl` function in PHP. `Curl` is a powerful PHP extension that allows you to send HTTP requests and retrieve remote data.

Here is a simple example code for checking whether a link exists:

$url = "http://www.example.com/";  
$curl = curl_init($url);  
curl_setopt($curl, CURLOPT_NOBODY, true);  
$result = curl_exec($curl);  

if ($result !== false) {  
  $statusCode = curl_getinfo($curl, CURLINFO_HTTP_CODE);  
  if ($statusCode == 200) {  
    echo "Link exists.";  
  } else {  
    echo "Link does not exist.";  
  }  
} else {  
  echo "Link does not exist.";  
}  

curl_close($curl);

This method uses `curl` to send a HEAD request, thereby avoiding the need to download the entire page content. If the link exists, a 200 status code will be returned. By checking the returned status code, you can determine whether the link exists.

Please note that this method may be disabled by the target website's security policies, or links may not be checked for other reasons. Therefore, it is advisable to handle any potential errors or exceptions with caution when using this approach.