[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.