PHP isset error fix

To resolve the issue in your `custom-listing-form.php` file, you can replace `isset()` with the `!== null` check.

For example, if your code looks like this:

<?php if (isset($some_array['key'])): ?>
    <!-- Some HTML -->
<?php endif; ?>

You should change it to:

<?php if ($some_array['key'] !== null): ?>
    <!-- Some HTML -->
<?php endif; ?>

This ensures that you're checking if the value is not `null`, which is the correct approach when you're dealing with an expression.

To fix this warning, you can use `isset()` or `array_key_exists()` to check if the key exists before attempting to access it. Here's how you can update your code:

### Using `isset()`:

<?php if (isset($some_array['submission'])): ?>
    <!-- Your code here -->
<?php endif; ?>

### Using `array_key_exists()`:

<?php if (array_key_exists('submission', $some_array)): ?>
    <!-- Your code here -->
<?php endif; ?>

`isset()` checks if the key exists and is not `null`, while `array_key_exists()` checks if the key exists regardless of its value (including `null`).

### Example for a form submission:

If you're dealing with form data (e.g., from `$_POST`), you can use this approach:

<?php if (isset($_POST['submission'])): ?>
    <!-- Process the form submission -->
<?php else: ?>
    <!-- Form not submitted yet -->
<?php endif; ?>

This ensures that you're only trying to access the `'submission'` key when it's set, avoiding the undefined array key warning.

To prevent this warning, you can use the `??` (null coalescing) operator or `isset()` to check if the key exists before trying to access it.

### Solution 1: Using the Null Coalescing Operator (`??`)
The null coalescing operator returns a default value if the key does not exist. Here's how you can use it:

<?php echo ($listing_data['provider_type'] ?? '') == 'Individual Provider' ? 'checked' : ''; ?>

This code will check if `$listing_data['provider_type']` exists. If it does, it will use its value; otherwise, it will use an empty string (`''`).

### Solution 2: Using `isset()`
Alternatively, you can use `isset()` to check if the key exists:

<?php echo (isset($listing_data['provider_type']) && $listing_data['provider_type'] == 'Individual Provider') ? 'checked' : ''; ?>

This code ensures that `'provider_type'` is defined before comparing its value.

### Explanation
– **`$listing_data['provider_type'] ?? ''`**: Returns `''` if `'provider_type'` is not set.
– **`isset($listing_data['provider_type'])`**: Checks if the key exists and is not `null`.

Both approaches prevent the **"Undefined array key"** warning and make your code more robust. Choose the one that best fits your coding style!