diff --git a/ext/standard/array.c b/ext/standard/array.c
index 0b184efc3998..504609f4c216 100644
--- a/ext/standard/array.c
+++ b/ext/standard/array.c
@@ -4275,15 +4275,20 @@ static zend_always_inline void php_array_merge_wrapper(INTERNAL_FUNCTION_PARAMET
 			ret = &args[0];
 		}
 		if (ret) {
-			if (HT_IS_PACKED(Z_ARRVAL_P(ret))) {
-				if (HT_IS_WITHOUT_HOLES(Z_ARRVAL_P(ret))) {
+			HashTable *ht = Z_ARRVAL_P(ret);
+			/* ZEND_LONG_MIN means no integer key yet, the next append uses index 0. */
+			zend_long next_free = ht->nNextFreeElement == ZEND_LONG_MIN ? 0 : ht->nNextFreeElement;
+
+			/* array_merge() renumbers integer keys from 0, so the next free index must match. */
+			if (HT_IS_PACKED(ht)) {
+				if (HT_IS_WITHOUT_HOLES(ht) && next_free == (zend_long) ht->nNumOfElements) {
 					RETURN_COPY(ret);
 				}
-			} else {
+			} else if (next_free == 0) {
 				bool copy = true;
 				zend_string *string_key;
 
-				ZEND_HASH_MAP_FOREACH_STR_KEY(Z_ARRVAL_P(ret), string_key) {
+				ZEND_HASH_MAP_FOREACH_STR_KEY(ht, string_key) {
 					if (!string_key) {
 						copy = false;
 						break;
diff --git a/ext/standard/tests/array/array_merge_empty_array_result.phpt b/ext/standard/tests/array/array_merge_empty_array_result.phpt
new file mode 100644
index 000000000000..bc480c884b7b
--- /dev/null
+++ b/ext/standard/tests/array/array_merge_empty_array_result.phpt
@@ -0,0 +1,34 @@
+--TEST--
+array_merge() with an empty argument returns an array with a fresh next free index
+--FILE--
+<?php
+// The result must behave like a fresh array: integer keys renumbered from 0.
+
+$a = [1, 2, 3];
+unset($a[2]);
+$r = array_merge($a, []);
+$r[] = 4;
+var_dump($r);
+
+$b = ['a' => 1];
+$b[] = 2;
+unset($b[0]);
+$r = array_merge([], $b);
+$r[] = 3;
+var_dump($r);
+?>
+--EXPECT--
+array(3) {
+  [0]=>
+  int(1)
+  [1]=>
+  int(2)
+  [2]=>
+  int(4)
+}
+array(2) {
+  ["a"]=>
+  int(1)
+  [0]=>
+  int(3)
+}