Fatal error: Call to a member function find() on string in (curl & htmldomparser)

I want to get html code and find from a website ,I'm using this code to do so but I'm getting error

"The simultaneous use of the curl and htmldomparser"

Fatal error: Call to a member function find() on string in C:\cod.php on line 46

Line ERROR:

  foreach ($content->find('div[class=postbody]') as $element)

I was able to get the full source using the login But the problem is :

   foreach ($content->find('div[class=postbody]') as $element)

this cod:

  include_once('/simple_html_dom.php');

  error_reporting(E_ALL);


  function vBulletinLogin($username,$password,$url,$posturl){
    $ch = curl_init(); 
    curl_setopt($ch, CURLOPT_HEADER,false);
    curl_setopt($ch, CURLOPT_COOKIEFILE, 'veri.txt'); 
    curl_setopt($ch, CURLOPT_COOKIEJAR, 'veri.txt'); 
    curl_setopt($ch, CURLOPT_RETURNTRANSFER,true);
    curl_setopt($ch, CURLOPT_COOKIESESSION,true);
    curl_setopt($ch, CURLOPT_SSL_VERIFYPEER,false);
    curl_setopt($ch, CURLOPT_SSL_VERIFYHOST,false);
    curl_setopt($ch, CURLOPT_FOLLOWLOCATION, false);
    curl_setopt($ch, CURLOPT_USERAGENT,$_SERVER['HTTP_USER_AGENT']);
    curl_setopt($ch, CURLOPT_REFERER, $url.'index.php');
    curl_setopt($ch, CURLOPT_URL, $url.'login.php?do=login'); 
    curl_setopt($ch, CURLOPT_POST, 1);
    curl_setopt($ch, CURLOPT_POSTFIELDS, "vb_login_username=$username&vb_login_password&s=&securitytoken=guest&do=login&vb_login_md5password=".md5($password)."&vb_login_md5password_utf=".md5($password));
    $exec = curl_exec($ch);

    curl_setopt($ch, CURLOPT_POST, 0);
    curl_setopt($ch, CURLOPT_REFERER, $url.'login.php?do=login'); 
    curl_setopt($ch, CURLOPT_URL, $url.'clientscript/vbulletin_global.js?v=373'); 
    $exec = curl_exec($ch);

    curl_setopt($ch, CURLOPT_REFERER, $url.'login.php?do=login');
    curl_setopt($ch, CURLOPT_URL, $url.'index.php'); 
    $exec = curl_exec($ch);

    curl_setopt($ch, CURLOPT_REFERER, $url.'index.php');
    curl_setopt($ch, CURLOPT_URL, $posturl); 
    $exec = curl_exec($ch);
    curl_close($ch);
    return $exec;



}

$content = vBulletinLogin('user','pass','url','url');
foreach ($content->find('div[class=postbody]') as $element)
    {

        echo $element;

    }