var_dump(json_encode("ab\xE0"));
var_dump(json_encode("ab\xE0", JSON_UNESCAPED_UNICODE));
?>
---EXPECT--
+--EXPECTF--
string(156) ""latin 1234 -\/ russian \u043c\u0430\u043c\u0430 \u043c\u044b\u043b\u0430 \u0440\u0430\u043c\u0443 specialchars \u0002 \b \n U+1D11E >\ud834\udd1e<""
string(100) ""latin 1234 -\/ russian мама мыла раму specialchars \u0002 \b \n U+1D11E >𝄞<""
-string(4) "null"
-string(4) "null"
+
+Warning: json_encode(): Invalid UTF-8 sequence in argument in %s on line %d
+bool(false)
+
+Warning: json_encode(): Invalid UTF-8 sequence in argument in %s on line %d
+bool(false)