Fix GH-15731: Prevent #[AllowDynamicProperties] on enums (#15733)

Zend/tests/allow_dynamic_properties_on_enum.phpt

@@ -0,0 +1,11 @@
+--TEST--
+#[AllowDynamicProperties] cannot be applied to enum (GH-15731)
+--FILE--
+<?php
+
+#[AllowDynamicProperties]
+enum Test {}
+
+?>
+--EXPECTF--
+Fatal error: Cannot apply #[AllowDynamicProperties] to enum Test in %s on line %d

Zend/zend_attributes.c

@@ -86,6 +86,11 @@ static void validate_allow_dynamic_properties(
 			ZSTR_VAL(scope->name)
 		);
 	}
+	if (scope->ce_flags & ZEND_ACC_ENUM) {
+		zend_error_noreturn(E_ERROR, "Cannot apply #[AllowDynamicProperties] to enum %s",
+			ZSTR_VAL(scope->name)
+		);
+	}
 	scope->ce_flags |= ZEND_ACC_ALLOW_DYNAMIC_PROPERTIES;
 }