title: Weird PHP error: 'Can't use function return value in write context'
body: <p>I'm getting this error and I can't make head or tail of it.</p>
  
  <p>The exact error message is:</p>
  
  <blockquote>
    <p>Fatal error: Can't use function return
    value in write context in
    /home/curricle/public_html/descarga/index.php
    on line 48</p>
  </blockquote>
  
  <p>Line 48 is:</p>
  
  <pre><code>if (isset($_POST('sms_code') == TRUE ) {
  </code></pre>
  
  <p>What could be going on here?</p>
  
  <p>Here's the full function:</p>
  
  <pre><code>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);
  }
  </code></pre>