Why is my code stopping on this statement [closed]

I'm trying to fix some code I got from a friend. It features a lot of if(!$var) lines and it results in a false positive. When I change it to isset($var) the code runs through. This is fine in some places, but it's not exactly the same.

I have read change documentation for PHP 5.1 and further on this topic. I believe this code is from an older version of PHP, but I cannot find where handling of this code has changed.

Is the code outdated, or am I missing something like a PHP extension?

examples of code with problem:

 function mailTo($to, $subject, $body, $meta, $from, $from_friendly){
      $mail = new PHPMailer;
      $mail->isSMTP();
      $mail->SMTPDebug = 0;
      $mail->Debugoutput = 'html';
      $mail->Host = $meta['smtp_server'];
      if(isset($meta['smtp_port']))
      $mail->Port = $meta['smtp_port'];
      $mail->SMTPAuth = true;
      $mail->Username = $meta['smip_uname'];
      $mail->Password = $meta['smip_pwd'];
      $mail->setFrom($from, $from_friendly);
      $mail->addReplyTo($from, $from_friendly);
      $mail->addAddress($to);
      $mail->Subject =$subject;
      $mail->msgHTML($body);
      $mail->AltBody = strip_tags($body);
      if (!$mail->send()) {
        return false;
      } else {
        return true;
      }
}

example 2:

public function getMessage($conn, $messageId) {
        $structure = imap_fetchstructure($conn, $messageId);
        // If message is not multipart
        if (!$structure->parts) {
            $this->getMessagePart($conn, $messageId, $structure, 0);
        }
        else {
            foreach ($structure->parts as $partno => $part) {
                $this->getMessagePart($conn, $messageId, $part, $partno+1);
            }
         }
    }