Validate private(namespace) visibility modifier combinations

Add validation for private(namespace) modifier usage at compile-time.

This commit adds clarifying comments and ensures redundant set visibility
is properly removed for private(namespace) private(namespace)(set).

Author: withinboredom

Zend/zend_API.c

@@ -4472,6 +4472,8 @@ ZEND_API zend_property_info *zend_declare_typed_property(zend_class_entry *ce, z
 				"Property with asymmetric visibility %s::$%s must have type",
 				ZSTR_VAL(ce->name), ZSTR_VAL(name));
 		}
+		/* Validate asymmetric visibility hierarchy: public < protected < private(namespace) < private
+		 * Set visibility must be equal to or more restrictive than get visibility. */
 		uint32_t get_visibility = zend_visibility_to_set_visibility(access_type & ZEND_ACC_PPP_MASK);
 		uint32_t set_visibility = access_type & ZEND_ACC_PPP_SET_MASK;
 		if (get_visibility > set_visibility) {
@@ -4482,7 +4484,8 @@ ZEND_API zend_property_info *zend_declare_typed_property(zend_class_entry *ce, z
 		/* Remove equivalent set visibility. */
 		if (((access_type & (ZEND_ACC_PUBLIC|ZEND_ACC_PUBLIC_SET)) == (ZEND_ACC_PUBLIC|ZEND_ACC_PUBLIC_SET))
 		 || ((access_type & (ZEND_ACC_PROTECTED|ZEND_ACC_PROTECTED_SET)) == (ZEND_ACC_PROTECTED|ZEND_ACC_PROTECTED_SET))
-		 || ((access_type & (ZEND_ACC_PRIVATE|ZEND_ACC_PRIVATE_SET)) == (ZEND_ACC_PRIVATE|ZEND_ACC_PRIVATE_SET))) {
+		 || ((access_type & (ZEND_ACC_PRIVATE|ZEND_ACC_PRIVATE_SET)) == (ZEND_ACC_PRIVATE|ZEND_ACC_PRIVATE_SET))
+		 || ((access_type & (ZEND_ACC_NAMESPACE_PRIVATE|ZEND_ACC_NAMESPACE_PRIVATE_SET)) == (ZEND_ACC_NAMESPACE_PRIVATE|ZEND_ACC_NAMESPACE_PRIVATE_SET))) {
 			access_type &= ~ZEND_ACC_PPP_SET_MASK;
 		}
 		/* private(set) properties are implicitly final. */

Zend/zend_compile.c

@@ -1037,6 +1037,7 @@ uint32_t zend_add_anonymous_class_modifier(uint32_t flags, uint32_t new_flag)
 uint32_t zend_add_member_modifier(uint32_t flags, uint32_t new_flag, zend_modifier_target target) /* {{{ */
 {
 	uint32_t new_flags = flags | new_flag;
+	/* Prevent combining visibility modifiers (public, protected, private, private(namespace)) */
 	if ((flags & ZEND_ACC_PPP_MASK) && (new_flag & ZEND_ACC_PPP_MASK)) {
 		zend_throw_exception(zend_ce_compile_error,
 			"Multiple access type modifiers are not allowed", 0);