I wrote the following to see if a submitted URL has a valid http response code and also if it responds quickly. 

Use the code like this:

<?php
$is_ok = http_response($url); // returns true only if http response code < 400
?>

The second argument is optional, and it allows you to check for  a specific response code

<?php
http_response($url,'400'); // returns true if http status is 400
?>

The third allows you to specify how long you are willing to wait for a response.

<?php
http_response($url,'200',3); // returns true if the response takes less than 3 seconds and the response code is 200
?>

<?php
function http_response($url, $status = null, $wait = 3)
{
        $time = microtime(true);
        $expire = $time + $wait;

        // we fork the process so we don't have to wait for a timeout
        $pid = pcntl_fork();
        if ($pid == -1) {
            die('could not fork');
        } else if ($pid) {
            // we are the parent
            $ch = curl_init();
            curl_setopt($ch, CURLOPT_URL, $url);
            curl_setopt($ch, CURLOPT_HEADER, TRUE);
            curl_setopt($ch, CURLOPT_NOBODY, TRUE); // remove body
            curl_setopt($ch, CURLOPT_RETURNTRANSFER, TRUE);
            $head = curl_exec($ch);
            $httpCode = curl_getinfo($ch, CURLINFO_HTTP_CODE);
            curl_close($ch);
            
            if(!$head)
            {
                return FALSE;
            }
            
            if($status === null)
            {
                if($httpCode < 400)
                {
                    return TRUE;
                }
                else
                {
                    return FALSE;
                }
            }
            elseif($status == $httpCode)
            {
                return TRUE;
            }
            
            return FALSE;
            pcntl_wait($status); //Protect against Zombie children
        } else {
            // we are the child
            while(microtime(true) < $expire)
            {
            sleep(0.5);
            }
            return FALSE;
        }
    }
?>

Hope this example helps.  It is not 100% tested, so any feedback [sent directly to me by email] is appreciated.

I needed to use cURL in a php script to download data using not only SSL for the server authentication but also for client authentication.
On a default install of Fedora, setting up the proper cURL parameters, I would get an error: 

$ php curl.php
 Peer certificate cannot be authenticated with known CA certificates

The data on http://curl.haxx.se/docs/sslcerts.html was most useful. Indeed, toward to bottom it tells you to add a missing link inside /etc/pki/nssdb to use the ca-bundle.crt file. You do it so:

# cd /etc/pki/nssdb
# ln -s /usr/lib64/libnssckbi.so libnssckbi.so

Now you can do client authentication, provided you have your certificate handy with:

<?php
$data = "<soap:Envelope>[...]</soap:Envelope>";
$tuCurl = curl_init();
curl_setopt($tuCurl, CURLOPT_URL, "https://example.com/path/for/soap/url/");
curl_setopt($tuCurl, CURLOPT_PORT , 443);
curl_setopt($tuCurl, CURLOPT_VERBOSE, 0);
curl_setopt($tuCurl, CURLOPT_HEADER, 0);
curl_setopt($tuCurl, CURLOPT_SSLVERSION, 3);
curl_setopt($tuCurl, CURLOPT_SSLCERT, getcwd() . "/client.pem");
curl_setopt($tuCurl, CURLOPT_SSLKEY, getcwd() . "/keyout.pem");
curl_setopt($tuCurl, CURLOPT_CAINFO, getcwd() . "/ca.pem");
curl_setopt($tuCurl, CURLOPT_POST, 1);
curl_setopt($tuCurl, CURLOPT_SSL_VERIFYPEER, 1);
curl_setopt($tuCurl, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($tuCurl, CURLOPT_POSTFIELDS, $data);
curl_setopt($tuCurl, CURLOPT_HTTPHEADER, array("Content-Type: text/xml","SOAPAction: \"/soap/action/query\"", "Content-length: ".strlen($data)));

$tuData = curl_exec($tuCurl);
if(!curl_errno($tuCurl)){
  $info = curl_getinfo($tuCurl);
  echo 'Took ' . $info['total_time'] . ' seconds to send a request to ' . $info['url'];
} else {
  echo 'Curl error: ' . curl_error($tuCurl);
}

curl_close($tuCurl);
echo $tuData;
?>

WARNING WARNING

In this example: http://php.net/manual/en/book.curl.php#102885 by "frank at interactinet dot com"

There's a small bug in

<?php

         ...

         elseif($status == $httpCode) 
            { 
                return TRUE; 
            } 

            return FALSE; 
            pcntl_wait($status); //Protect against Zombie children 
        } else { 
            // we are the child 
            while(microtime(true) < $expire) 

         ...

?>

The code will immediately leave the function at the `return`, and pcntl_wait() will NEVER be executed, under any circumstances.

I can't see any other issues with this function however.

Hey I modified script for php 5. Also I add support server auth. and fixed some little bugs on the script.

[EDIT BY danbrown AT php DOT net: Original was written by (unlcuky13 AT gmail DOT com) on 19-APR-09.  The following note was included:
Below is the my way of using through PHP 5 objecte oriented encapsulation to make thing easier.]

<?php
 class mycurl {
     protected $_useragent = 'Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1';
     protected $_url;
     protected $_followlocation;
     protected $_timeout;
     protected $_maxRedirects;
     protected $_cookieFileLocation = './cookie.txt';
     protected $_post;
     protected $_postFields;
     protected $_referer ="http://www.google.com";

     protected $_session;
     protected $_webpage;
     protected $_includeHeader;
     protected $_noBody;
     protected $_status;
     protected $_binaryTransfer;
     public    $authentication = 0;
     public    $auth_name      = '';
     public    $auth_pass      = '';

     public function useAuth($use){
       $this->authentication = 0;
       if($use == true) $this->authentication = 1;
     }

     public function setName($name){
       $this->auth_name = $name;
     }
     public function setPass($pass){
       $this->auth_pass = $pass;
     }

     public function __construct($url,$followlocation = true,$timeOut = 30,$maxRedirecs = 4,$binaryTransfer = false,$includeHeader = false,$noBody = false)
     {
         $this->_url = $url;
         $this->_followlocation = $followlocation;
         $this->_timeout = $timeOut;
         $this->_maxRedirects = $maxRedirecs;
         $this->_noBody = $noBody;
         $this->_includeHeader = $includeHeader;
         $this->_binaryTransfer = $binaryTransfer;

         $this->_cookieFileLocation = dirname(__FILE__).'/cookie.txt';

     }

     public function setReferer($referer){
       $this->_referer = $referer;
     }

     public function setCookiFileLocation($path)
     {
         $this->_cookieFileLocation = $path;
     }

     public function setPost ($postFields)
     {
        $this->_post = true;
        $this->_postFields = $postFields;
     }

     public function setUserAgent($userAgent)
     {
         $this->_useragent = $userAgent;
     }

     public function createCurl($url = 'nul')
     {
        if($url != 'nul'){
          $this->_url = $url;
        }

         $s = curl_init();

         curl_setopt($s,CURLOPT_URL,$this->_url);
         curl_setopt($s,CURLOPT_HTTPHEADER,array('Expect:'));
         curl_setopt($s,CURLOPT_TIMEOUT,$this->_timeout);
         curl_setopt($s,CURLOPT_MAXREDIRS,$this->_maxRedirects);
         curl_setopt($s,CURLOPT_RETURNTRANSFER,true);
         curl_setopt($s,CURLOPT_FOLLOWLOCATION,$this->_followlocation);
         curl_setopt($s,CURLOPT_COOKIEJAR,$this->_cookieFileLocation);
         curl_setopt($s,CURLOPT_COOKIEFILE,$this->_cookieFileLocation);

         if($this->authentication == 1){
           curl_setopt($s, CURLOPT_USERPWD, $this->auth_name.':'.$this->auth_pass);
         }
         if($this->_post)
         {
             curl_setopt($s,CURLOPT_POST,true);
             curl_setopt($s,CURLOPT_POSTFIELDS,$this->_postFields);

         }

         if($this->_includeHeader)
         {
               curl_setopt($s,CURLOPT_HEADER,true);
         }

         if($this->_noBody)
         {
             curl_setopt($s,CURLOPT_NOBODY,true);
         }
         /*
         if($this->_binary)
         {
             curl_setopt($s,CURLOPT_BINARYTRANSFER,true);
         }
         */
         curl_setopt($s,CURLOPT_USERAGENT,$this->_useragent);
         curl_setopt($s,CURLOPT_REFERER,$this->_referer);

         $this->_webpage = curl_exec($s);
                   $this->_status = curl_getinfo($s,CURLINFO_HTTP_CODE);
         curl_close($s);

     }

   public function getHttpStatus()
   {
       return $this->_status;
   }

   public function __tostring(){
      return $this->_webpage;
   }
}
?>

[EDIT BY danbrown AT php DOT net: Contains a bugfix supplied by "roetsch.beni at googlemail (dot) com" on 02-AUG-09, with the following note: "Fixes the bugfix: 417 bug at lighthttp server."]

Please note that new versions of curl is using http2 as default, so if you are having some strange errors, 0 http status codes, etc, please explicitly specify the http version in your code.

CURL failed with PHP5.3 and Apache2.2.X on my Windows 7 machine. 

It turns out that it's not enough to copy the two dll's mentioned (libeay32 and sslea32) from the php folder into your system32 folder. You HAVE TO UNBLOCK THESE TWO FILES.

Right click the file, select unblock, for each one. Then restart Apache.

Another very handy security feature added into Windows.

Sharing is caring, handles included.

<?php

$url_one = "php.net";
$url_two = "";

$user_agent = 'Mozilla HotFox 1.0';

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $url_one.$url_two);
curl_setopt($ch, CURLOPT_USERAGENT, $user_agent);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_FOLLOWLOCATION, 1);
curl_setopt($ch, CURLOPT_HEADER, 0);
curl_setopt($ch, CURLOPT_NOBODY, 0);
curl_setopt($ch, CURLOPT_TIMEOUT, 30);
$res = curl_exec($ch);
curl_close($ch);

$url_two = "lazyphp.net";
$url_one = "";
$res_two = curl_exec($ch);
curl_close($ch);

?>

Here you have a function that I use to get the content of a URL using cURL:

function getUrlContent($url){
 $ch = curl_init();
 curl_setopt($ch, CURLOPT_URL, $url);
 curl_setopt($ch, CURLOPT_USERAGENT, 'Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.1.4322)');
 curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
 curl_setopt($ch, CURLOPT_CONNECTTIMEOUT, 5);
 curl_setopt($ch, CURLOPT_TIMEOUT, 5);
 $data = curl_exec($ch);
 $httpcode = curl_getinfo($ch, CURLINFO_HTTP_CODE);
 curl_close($ch);
 return ($httpcode>=200 && $httpcode<300) ? $data : false;
}

The source comes from this website:

http://softontherocks.blogspot.com/2014/11/descargar-el-contenido-de-una-url.html

After a lot of frustration with the fact that nobody has documented which curl commandline options go with which library functions, I discovered that the curl commandline will tell you (in the form of a C program) if you add `--libcurl foo.c`

If you've been struggling with trying to figure out how to get your fancy curl commandline to work in PHP, this makes it a breeze!