Vkontakte authorization how to fix an error?

Question

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Hello!
Swearing at line 19 gives an error Fatal error: Call to undefined function curl() in .

<?php
require "include/bittorrent.php";
if($CURUSER){
    header("Location: $DEFAULTBASEURL/");   
} else {
dbconn(false);
stdhead("Авторизация через ВКонтакте");
function bark($msg) {
    global $tracker_lang;
    stdhead();
    stdmsg($tracker_lang['error'], $msg, 'error');
    stdfoot();
    exit;
}
$mail = $_POST['user']; //E-mail  
$pass = $_POST['pass']; //пароль 
$url_auth='https://oauth.vk.com/token?grant_type=password&client_id=2274003&client_secret=hHbZxrka2uZ6jB1inYsH&username='.$mail.'&password='.$pass; 

$content=curl($url_auth,'','',''); 
#функция декодирования масива JSON 
$json=json_decode($content, true); 
if(isset($json[error_description])){ 
    bark("Авторизация невозможна!");
}else { 
if (!validemail($mail))
    bark("Это не похоже на реальный email адрес.");
// check if email addy is already in use
$a = (@mysql_fetch_row(@sql_query("SELECT COUNT(*) FROM users WHERE email=".sqlesc($mail)))) or die(mysql_error());
if ($a[0] != 0)
    bark("E-mail адрес ".htmlspecialchars($mail)." уже зарегистрирован в системе.");
    
    $url_p="https://api.vk.com/method/users.get?user_id=".$json[user_id]."&v=5.21&access_token=".$json[access_token]; 
    $res=curl($url_p,'','',''); 
        $json_auth=json_decode($res, true); 
        $users = get_row_count("users");
        $secret = mksecret();
        $wantpasshash = md5($secret . $pass . $secret);
        $wantusername = 'vk'.$json_auth[response][0][id].'_'.rus2translit(utf8_win($json_auth[response][0][first_name]));
        $status = 'confirmed';
# REG    
# MYSQL - ALTER TABLE `users` ADD `login` varchar(40) NOT NULL default '';        
    $ret = sql_query("INSERT INTO users (username, passhash, secret, avatar, email, status, added, login) VALUES (
    " .    implode(",", array_map("sqlesc", array($wantusername, $wantpasshash, $secret, $DEFAULTBASEURL.'/pic/avavk.png', $mail, $status))).", 
    '". get_date_time() ."', 'vk')") or sqlerr(__FILE__, __LINE__);
# LOGIN
    $id = mysql_insert_id();
    logincookie($id, $wantpasshash);
    header("Location: $DEFAULTBASEURL/");     
} 
stdfoot();
}
?>

CURL is installed on the hosting. Help me fix it please.