Weird PHP error: 'Can't use function return value in write context'

I'm getting this error and I can't make head or tail of it.

The exact error message is:

Fatal error: Can't use function return value in write context in /home/curricle/public_html/descarga/index.php on line 48

Line 48 is:

if (isset($_POST('sms_code') == TRUE ) {

What could be going on here?

Here's the full function:

function validate_sms_code() {

    $state = NOTHING_SUBMITED;

    if (isset($_POST('sms_code') == TRUE ) {
        $sms_code = clean_up($_POST('sms_code'));
        $return_code = get_sepomo_code($sms_code);

        switch($return_code) {

          case 1:
            //no error
            $state = CORRECT_CODE;
            break;

          case 2:
            // code already used
            $state = CODE_ALREADY_USED;
            break;

          case 3:
            // wrong code
            $state = WRONG_CODE;
            break;

          case 4:
            // generic error
            $state = UNKNOWN_SEPOMO_CODE;
            break;

          default:
            // unknown error
            $state = UNKNOWN_SEPOMO_CODE;
            throw new Exception('Unknown sepomo code: ' . $return_code);
            break;
        }

    } else {
        $state = NOTHING_SUBMITED;
    }
    dispatch_on_state($state);
}

This also happens when using empty on a function return:

!empty(trim($someText)) and doSomething()

because empty is not a function but a language construct (not sure), and it only takes variables:

Right:

empty($someVar)

Wrong:

empty(someFunc())

Since PHP 5.5, it supports more than variables. But if you need it before 5.5, use trim($name) == false. From empty documentation.

You mean

if (isset($_POST['sms_code']) == TRUE ) {

though incidentally you really mean

if (isset($_POST['sms_code'])) {

if (isset($_POST('sms_code') == TRUE ) {

change this line to

if (isset($_POST['sms_code']) == TRUE ) {

You are using parentheseis () for $_POST but you wanted square brackets []

:)

OR

if (isset($_POST['sms_code']) && $_POST['sms_code']) { 
//this lets in this block only if $_POST['sms_code'] has some value