diff --git a/ext/standard/string.c b/ext/standard/string.c
index f21c9be8a7bd2..36903b3c5c7b9 100644
--- a/ext/standard/string.c
+++ b/ext/standard/string.c
@@ -4926,37 +4926,52 @@ PHP_FUNCTION(setlocale)
 {
 	zend_long cat;
 	zval *args = NULL;
-	int num_args;
+	uint32_t num_args;
+	ALLOCA_FLAG(use_heap);
 
 	ZEND_PARSE_PARAMETERS_START(2, -1)
 		Z_PARAM_LONG(cat)
 		Z_PARAM_VARIADIC('+', args, num_args)
 	ZEND_PARSE_PARAMETERS_END();
 
+	zend_string **strings = do_alloca(sizeof(zend_string *) * num_args, use_heap);
+
+	for (uint32_t i = 0; i < num_args; i++) {
+		if (UNEXPECTED(Z_TYPE(args[i]) != IS_ARRAY && !zend_parse_arg_str(&args[i], &strings[i], false, i + 2))) {
+			zend_wrong_parameter_type_error(i + 2, Z_EXPECTED_ARRAY_OR_STRING, &args[i]);
+			goto out;
+		}
+	}
+
 	for (uint32_t i = 0; i < num_args; i++) {
 		if (Z_TYPE(args[i]) == IS_ARRAY) {
 			zval *elem;
 			ZEND_HASH_FOREACH_VAL(Z_ARRVAL(args[i]), elem) {
 				zend_string *result = try_setlocale_zval(cat, elem);
 				if (EG(exception)) {
-					RETURN_THROWS();
+					goto out;
 				}
 				if (result) {
-					RETURN_STR(result);
+					RETVAL_STR(result);
+					goto out;
 				}
 			} ZEND_HASH_FOREACH_END();
 		} else {
-			zend_string *result = try_setlocale_zval(cat, &args[i]);
+			zend_string *result = try_setlocale_str(cat, strings[i]);
 			if (EG(exception)) {
-				RETURN_THROWS();
+				goto out;
 			}
 			if (result) {
-				RETURN_STR(result);
+				RETVAL_STR(result);
+				goto out;
 			}
 		}
 	}
 
-	RETURN_FALSE;
+	RETVAL_FALSE;
+
+out:
+	free_alloca(strings, use_heap);
 }
 /* }}} */
 
diff --git a/ext/standard/tests/strings/gh18823_strict.phpt b/ext/standard/tests/strings/gh18823_strict.phpt
new file mode 100644
index 0000000000000..80b21d2093172
--- /dev/null
+++ b/ext/standard/tests/strings/gh18823_strict.phpt
@@ -0,0 +1,19 @@
+--TEST--
+GH-18823 (setlocale's 2nd and 3rd argument ignores strict_types) - strict mode
+--FILE--
+<?php
+declare(strict_types=1);
+try {
+    setlocale(LC_ALL, 0, "0");
+} catch (TypeError $e) {
+    echo $e->getMessage(), "\n";
+}
+try {
+    setlocale(LC_ALL, "0", 0);
+} catch (TypeError $e) {
+    echo $e->getMessage(), "\n";
+}
+?>
+--EXPECT--
+setlocale(): Argument #2 ($locales) must be of type array|string, int given
+setlocale(): Argument #3 must be of type array|string, int given
diff --git a/ext/standard/tests/strings/gh18823_weak.phpt b/ext/standard/tests/strings/gh18823_weak.phpt
new file mode 100644
index 0000000000000..bc9611098a8db
--- /dev/null
+++ b/ext/standard/tests/strings/gh18823_weak.phpt
@@ -0,0 +1,32 @@
+--TEST--
+GH-18823 (setlocale's 2nd and 3rd argument ignores strict_types) - weak mode
+--INI--
+error_reporting=E_ALL
+--FILE--
+<?php
+class MyStringable {
+    public function __toString(): string {
+        return 'foo';
+    }
+}
+
+class MyStringableThrow {
+    public function __toString(): string {
+        throw new Error('no');
+    }
+}
+
+setlocale(LC_ALL, 0, "0");
+setlocale(LC_ALL, "0", 0);
+setlocale(LC_ALL, null);
+setlocale(LC_ALL, new MyStringable);
+
+try {
+    setlocale(LC_ALL, new MyStringableThrow);
+} catch (Error $e) {
+    echo $e->getMessage(), "\n";
+}
+?>
+--EXPECTF--
+Deprecated: setlocale(): Passing null to parameter #2 ($locales) of type string is deprecated in %s on line %d
+no
diff --git a/ext/standard/tests/strings/setlocale_variation4.phpt b/ext/standard/tests/strings/setlocale_variation4.phpt
index ca469759dbd15..a2b236c146c8c 100644
--- a/ext/standard/tests/strings/setlocale_variation4.phpt
+++ b/ext/standard/tests/strings/setlocale_variation4.phpt
@@ -28,7 +28,7 @@ var_dump($locale_info_before);
 
 //Testing setlocale()  by giving locale = null
 echo "Setting system locale, category = LC_ALL and locale = null\n";
-setlocale(LC_ALL, null);
+@setlocale(LC_ALL, null);
 
 echo "Locale info, after setting the locale\n";
 //Returns Current locale,after executing setlocale().