Yes, correct PHP sends the Content-Type charset header value automatically, if you not set it directly in into your response object. PHP uses the value from this ini setting:
ini_set('default_charset', 'utf-8');
If you set this value to an empty string, PHP will not add charset=UTF-8 to the response.
ini_set('default_charset', '');
In the long run, it would be better to explicitly define your specific charset. For example, if your XML content is not UTF-8, it is better to send it instead of omitting it. For example: