Why the empty statement can only control user

Question

if ($user==""||$pwd==""||$pwd_confirm==""){

echo "<script>alert('Please confirm the integrity of the information ');history.go(-1);</script>";

}

elseif ($code!=$_SESSION["ver_code"] ){

     

                                    echo "<script>alert('Verification code is incorrect');history.go(-1);</script>";

  }

Answer 1

Please check the instructions for using the logical operation "||"