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 ) {

Anybody knows what's going on???

PS Here's the full function, in case it helps:

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);
}

Answer

You mean

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

though incidentally you really mean

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