Hers's is the tutorial to submit form via curl.

I am taking example of log in form for say xyz.com site.

$ch = curl_init();
if($ch){
   $url = 'http://www.xyz.com/logmein.php';
    $postdata = array('LoginButton' => "Logon",
                       'PasswordTextbox'   => "mypasswordsafest",
                       'UsernameTextbox'   => "memyselfirine",
                       'secret_string' => "shhhmysecretstring"
                     );
    curl_setopt($ch, CURLOPT_URL, $url);
    curl_setopt($ch, CURLOPT_CONNECTTIMEOUT, 15);
    curl_setopt($ch, CURLOPT_FOLLOWLOCATION, true);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    curl_setopt($ch, CURLOPT_POST, 1);
    curl_setopt($ch, CURLOPT_POSTFIELDS, $postdata);
    curl_setopt($ch, CURLOPT_COOKIEFILE, 'cookies.txt'); 
    curl_setopt($ch, CURLOPT_COOKIEJAR, 'cookies.txt'); 
    curl_setopt($ch, CURLOPT_USERAGENT, 'Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1');
    curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, FALSE);     
    curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 2);   

    curl_exec($ch);
    $headers = curl_getinfo($ch);
    $errors = curl_error($ch);
    if($headers['http_code'] != 200) { 
            /* Error Code here */
    }else {
           /* do whatever */
    }
}

Let me explain what curl options do (after initializing)

curl_setopt($ch, CURLOPT_URL, $url);

setup the url you want to load, in this case we have login page url, you can also set this target url in curl_init function.

curl_setopt($ch, CURLOPT_CONNECTTIMEOUT, 15);

You dont want to keep curl script waiting for indefinite time, here 15 indicates a 15 sec period.

curl_setopt($ch, CURLOPT_FOLLOWLOCATION, true);

Redirects the action mentioned in the header after submission

curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);

Do not display the output but return it as an string.

curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);

Send data as post field

curl_setopt($ch, CURLOPT_POSTFIELDS, $postdata);

You need to send username and password atleast, send it in form of array, here I used $postdata array for this.

curl_setopt($ch, CURLOPT_COOKIEJAR, 'cookies.txt');

Set the file where cookie should be stored

 curl_setopt($ch, CURLOPT_COOKIEFILE, 'cookies.txt'); 

A separate setting for curl to send cookie back to the server

curl_setopt($ch, CURLOPT_USERAGENT, 'Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1');

The Value you want to appear in the weblogs

curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, FALSE);

Set this to false, if you want your request not to trigger in case of invalid certificate

curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 2); 

Set this to false , if you want your request not to trigger in case of name in the remote server's certificate

check these params and execute curl via curl_exec($ch);