"\u0628\u0633\u0645 \u0627\u0644\u0644\u0647"
and "بسم الله"
are equivalent in JSON.
PHP just defaults to using Unicode escapes instead of literals for multibyte characters.
You can specify otherwise with JSON_UNESCAPED_UNICODE (providing you are using PHP 5.4 or later).
json_encode('بسم الله', JSON_UNESCAPED_UNICODE);