[PHP] Enum assigned as a reference must produce error

### Apache NetBeans version

Apache NetBeans 16

### What happened

Enum values can't be changed, therefore assigning enum as a reference must produce error.

### How to reproduce

```php
enum Suit: string {
case Hearts = 'H';
case Diamonds = 'D';
case Clubs = 'C';
case Spades = 'S';
}

$card = Suit::Clubs;
$ref = &$card->value; // this statement must display error
$val = $card->value; // OK (not referencing)
```

### Did this work correctly in an earlier version?

No / Don't know

### Operating System

Windows 10

### JDK

Java: 14.0.1; Java HotSpot(TM) 64-Bit Server VM 14.0.1+7 Runtime: Java(TM) SE Runtime Environment 14.0.1+7

### Apache NetBeans packaging

Apache NetBeans provided installer

### Anything else

_No response_

### Are you willing to submit a pull request?

No

### Code of Conduct

Yes

Start by reproducing the PHP enum example in Apache NetBeans 16, focusing on the reference assignment to the enum value. Done means the reference assignment displays an error while direct, non-reference access to the value remains valid.

Written by the indexing model from the issue text.