[php-src] Issue #20663: Need a Built-in `is_assigned()` to distinguish between declared-but-uninitialized and assigned-null

Issue: https://github.com/php/php-src/issues/20663
Author: HichemTab-tech

### Description

### TL;DR

PHP has no built-in way to check whether a variable or property has actually been **assigned**, as opposed to simply being `null` or not existing. `isset()` can’t tell the difference between “assigned null” and “not assigned,” and nothing else works consistently. A function like `is_assigned()` would solve this.

### **Full**

Right now PHP has no reliable way to check if a variable, array key, or object property has *actually been assigned*, independent of its value.

`isset()` doesn’t help, because it returns false for both cases:

* when the variable/property is not defined
* when it’s defined but explicitly set to `null`

`property_exists()` doesn’t help either, because it reports properties as existing even if they were never assigned (just declared on the class).

`get_object_vars()` kind of works for objects, because it only lists assigned properties, even if they're assigned `null`. But this only works for objects, not variables. Reflection can detect uninitialized *typed* properties, but not untyped ones, and not variables or arrays.

So there’s no general solution in PHP today to tell the difference between:

* a variable/property that was never assigned
* a variable/property that was assigned `null`

This distinction matters when building DTOs, serializers, hydrators, ORMs, or anything where “user assigned null” is different from “value was never provided.”

It would help a lot if PHP had something like:

```php
is_assigned($var);
// or
is_assigned($myClass->prop);
```

which returns true if the variable/property/array key has ever been assigned, even if the assigned value is null, and false if it was never assigned or doesn’t exist.

This wouldn’t break anything and would fill a long-standing gap in the language where userland code simply cannot make this distinction.