$url = 'http://google.com';
$not_url = 'stp://google.com';

if (@file_get_contents($url)): echo "Found '$url'!";
else: echo "Can't find '$url'.";
endif;
if (@file_get_contents($not_url)): echo "Found '$not_url!";
else: echo "Can't find '$not_url'.";
endif;

// Found 'http://google.com'!Can't find 'stp://google.com'.

检查URL是否有效的其他方法可以是:

<?php

  if (isValidURL("http://www.gimepix.com")) {
      echo "URL is valid...";
  } else {
      echo "URL is not valid...";
  }

  function isValidURL($url) {
      $file_headers = @get_headers($url);
      if (strpos($file_headers[0], "200 OK") > 0) {
         return true;
      } else {
        return false;
      }
  }
?>

简单的方法是卷曲(和更快)

<?php
$mylinks="http://site.com/page.html";
$handlerr = curl_init($mylinks);
curl_setopt($handlerr,  CURLOPT_RETURNTRANSFER, TRUE);
$resp = curl_exec($handlerr);
$ht = curl_getinfo($handlerr, CURLINFO_HTTP_CODE);


if ($ht == '404')
     { echo 'OK';}
else { echo 'NO';}

?>

在检查报头是否有404错误时，需要考虑的一件事是站点不会立即生成404错误。

很多网站会检查PHP/ASP(等等)源代码中是否存在某个页面，然后将您转到404页面。在这些情况下，头基本上是由生成的404头扩展的。在这种情况下，404错误不会出现在头文件的第一行，而是第10行。

$array = get_headers($url);
$string = $array[0];
print_r($string) // would generate:

Array ( 
[0] => HTTP/1.0 301 Moved Permanently 
[1] => Date: Fri, 09 Nov 2018 16:12:29 GMT 
[2] => Server: Apache/2.4.34 (FreeBSD) LibreSSL/2.7.4 PHP/7.0.31 
[3] => X-Powered-By: PHP/7.0.31 
[4] => Set-Cookie: landing=%2Freed-diffuser-fig-pudding-50; path=/; HttpOnly 
[5] => Location: /reed-diffuser-fig-pudding-50/ 
[6] => Content-Length: 0 
[7] => Connection: close 
[8] => Content-Type: text/html; charset=utf-8 
[9] => HTTP/1.0 404 Not Found 
[10] => Date: Fri, 09 Nov 2018 16:12:29 GMT 
[11] => Server: Apache/2.4.34 (FreeBSD) LibreSSL/2.7.4 PHP/7.0.31 
[12] => X-Powered-By: PHP/7.0.31 
[13] => Set-Cookie: landing=%2Freed-diffuser-fig-pudding-50%2F; path=/; HttpOnly 
[14] => Connection: close 
[15] => Content-Type: text/html; charset=utf-8 
) 

function url_exists($url) {
    $headers = @get_headers($url);
    return (strpos($headers[0],'200')===false)? false:true;
}

$url = 'http://google.com';
$not_url = 'stp://google.com';

if (@file_get_contents($url)): echo "Found '$url'!";
else: echo "Can't find '$url'.";
endif;
if (@file_get_contents($not_url)): echo "Found '$not_url!";
else: echo "Can't find '$not_url'.";
endif;

// Found 'http://google.com'!Can't find 'stp://google.com'.