cURL

cURL stands for Client URL.libcurl is a portable library that provides an easy interface to the cURL functionality.
Basically what it does is connects to other websites and gets information.
cURL is a command line tool used to transfer files with URL syntax
The data transfer protocols supported by cURL include HTTP, HTTPS, FTP, FTPS, GOPHER, LDAP, DICT, TELNET and FILE.

PHP cURL allows you to read websites, make automated logins, upload files and many more. I have used
it for creating Twitter API, URL shortening, Facebook API and also for downloading feed updaters for
a web site.
Using it is a 4 step process.

1. Initialize the curl,

   $ch = curl_init();

2. Set the options for curl. This is where all the action takes place

   curl_setopt($ch, CURLOPT_HEADER, 1); //Display the server response header.
   curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);//This tells cURL to return the data instead of displaying it

   By default, the result is displayed in the browser.   

3. Execute and fetch result

   $yahoo=curl_exec($ch); //The result is stored in $yahoo.

4. Free the curl handle

   curl_close($ch);

A sample server response header

HTTP Headers for www.boatbuys.ca
==============

HTTP/1.1 302 Found
Date: Tue, 19 Jul 2011 14:10:44 GMT
Server: Apache/2.2.17 (Unix) mod_ssl/2.2.17 OpenSSL/0.9.8e-fips-rhel5 mod_bwlimited/1.4 PHP/5.3.4
X-Powered-By: PHP/5.3.4
Set-Cookie: PHPSESSID=f629dd2154a7ea018812e52f5d5541ed; path=/
Expires: Thu, 19 Nov 1981 08:52:00 GMT
Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
Pragma: no-cache
Location: http://www.boatbuys.com/
Vary: Accept-Encoding,User-Agent
Connection: close
Content-Type: text/html; charset=utf-8

The problem with using cURL in the above example is that all the info must be first fed to the memory.
If the info is large, this can be a hassle. Instead, we can use a file to store the info using the CURLOPT_FILE option

Steps:
      -Create a new file pointer using fopen
      -Pass the file pointer to cURL
      -Perform the request

eg:  

    <?php
    $url  = 'http://www.php-pranthan.blogspot.com/a-large-file.zip';  //source
    $path = 'file.zip';  //destination file location

    $fp = fopen($path, 'w');

    $ch = curl_init($url);
    curl_setopt($ch, CURLOPT_FILE, $fp);
    curl_setopt ($ch, CURLOPT_FOLLOWLOCATION, 1);//Follow any "Location:" header that the server sends as part of the HTTP header

    $data = curl_exec($ch);

    curl_close($ch);
    fclose($fp);
    ?>

For more info:
http://ca2.php.net/curl_setopt