Assigns True but not false?

In this if statement, if the condition is true, then it will assign true to the array (as I want it to). If the condition evaluates to false, it assigns nothing. Any idea why it won't assign false? If I switch the FALSE boolean value to the number 0, that will get assigned. Seems kinda strange... $this->m_arrQuesInfo[$this->m_itemID]['blnVacPromo'] = ( ( !empty($_POST['vac_promo']) ) && ( $_POST['vac_promo'] == 'on' ) ) ? TRUE : FALSE;