require_once('classes/class.smsquick.php');

$username = "someuser";
$password = "somepass";

$api = new SmsQuick($username, $password);

//uncommenting the line below returns undefined_index @ line 312 and 282
//$available_credits = $api->checkBalance();

//var_dump($api); //used for checking

public function checkBalance() {
    $vars = array(
        'username' => $this->api_username,
        'password' => $this->api_password,
        'action' => 'balance',
    );

    $retval = $this->executeApiRequest($vars);
    list(, $response) = array_values(reset($retval)); // line 282

    return (int) $response;
}

/**
 * Helper method to execute an API request.
 *
 * @param array $vars
 *   Data to POST to SMS gateway API endpoint.
 *
 * @return array
 *   Response from SMS gateway.
 */
public function executeApiRequest($vars) {
    // Basic validation on the authentication details
    foreach ($vars as $key => $value) {
        switch ($key) {
            case 'username':
            case 'password':
                if (empty($value)) {
                    throw new Exception('API username or password not specified.');
                }
                break;
        }
    }

    $data = $this->preparePostData($vars);
    $retval = $this->executePostRequest($data);

    list($status, $response) = explode(':', $retval); // line 312
    if ($status == 'ERROR') {
        throw new Exception(strtr('There was an error with this request: !error.', array('!error' => $response)));
    }

    $data = array();
    $lines = explode("n", $retval);
    foreach (array_filter($lines) as $i => $line) {
        $line = trim($line);
        $data[$i] = explode(':', $line);
    }

    return $data;
}

protected function preparePostData($data) {
    $post_data = array();
    foreach ($data as $key => $value) {
        switch ($key) {
            case 'to':
                // Support multiple phone numbers.
                $value = implode(',', array_unique($value));
                break;
        }
        $post_data[] = $key . '=' . rawurlencode($value);
    }

    return implode('&', $post_data);
}

protected function executePostRequest($data) {
    $ch = curl_init($this->api_endpoint);
    curl_setopt($ch, CURLOPT_POST, TRUE);
    curl_setopt($ch, CURLOPT_POSTFIELDS, $data);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, TRUE);
    $retval = curl_exec($ch);
    curl_close($ch);
    return $retval;
}

reset($retval);
list(, $response) = array_values(reset($retval)); // line 282