PHP E_NOTICE best practice - Is it bad to check an undefined var for a value?

With E_NOTICE error messages enabled, PHP doesn't like the following code, unless the variables $mdDialog and $mdToast have already been defined:

if ($mdDialog || $mdToast) {
    $ngMaterial = true;
}

To avoid E_NOTICE error, I must write:

if (isset($mdDialog) || isset($mdToast)) {
    $ngMaterial = true;
}

The problem is that, with the above code, if I have a $mdDialog = false; line somewhere earlier, the statement will be truthy, which is not the idea. To avoid this, I'd have to write:

if ((isset($mdDialog) && $mdDialog) || (isset($mdToast) && $mdToast)) {
    $ngMaterial = true;
}

And this is sooo much longer and feels excessive, only to avoid the E_NOTICE message.

So, the question is, should I care about these E_NOTICE messages? Is there anything bad with checking the value of a variable that may not exist?

Solution

empty can be useful in this case. Like below:

if (!empty($mdDialog) || !empty($mdToast)) {
    $ngMaterial = true;
}