diff --git a/ext/zend_test/tests/gh11423.phpt b/ext/zend_test/tests/gh11423.phpt
new file mode 100644
index 0000000000000..3c153229b3976
--- /dev/null
+++ b/ext/zend_test/tests/gh11423.phpt
@@ -0,0 +1,29 @@
+--TEST--
+GH-11423 (internal constants have their namespace lowercased)
+--EXTENSIONS--
+zend_test
+--FILE--
+<?php
+
+$re = new \ReflectionExtension('zend_test');
+var_dump($re->getConstants());
+
+define('NS1\ns2\Const1','value1');
+var_dump(get_defined_constants(true)["user"]);
+
+?>
+--EXPECT--
+array(4) {
+  ["ZEND_TEST_DEPRECATED"]=>
+  int(42)
+  ["ZEND_CONSTANT_A"]=>
+  string(6) "global"
+  ["zendtestns2\ZEND_CONSTANT_A"]=>
+  string(10) "namespaced"
+  ["zendtestns2\zendsubns\ZEND_CONSTANT_A"]=>
+  string(10) "namespaced"
+}
+array(1) {
+  ["ns1\ns2\Const1"]=>
+  string(6) "value1"
+}