注意：使用未定义的常量消息 -

This should work now how it is but it just keeps posting me this error:

Notice: Use of undefined constant Messages

Code:

<?php
    if ($user_id>Messages == 1) {
     echo $user_id->Usermessage;
    } elseif($user_id>Messages == 0) {
       echo "No Messages";
    }
?>

you missing "-" when your type $user_id->Usermessage instead of $user_id>Usermessage. So, It should be as follow

<?php
    if ($user_id>Messages == 1) {
     echo $user_id->Usermessage;
    } elseif($user_id->Messages == 0) {
       echo "No Messages";
    }
?>

You've missed the hyphen - in your elseif statement. Should be elseif($user_id->Messages == 0)