mb_output_handler() is ob_start() callback function. mb_output_handler() converts characters in output buffer from internal character encoding to HTTP output character encoding.
4.1.0 or later version, this handler adds charset HTTP header when following conditions are met:
http_outputsetting is other than pass
contents : Output buffer contents
status : Output buffer status
Return Value: String converted
If you want to output some binary data such as image from PHP script with PHP 4.3.0 or later, Content-Type: header must be send using header() before any binary data was send to client (e.g. header("Content-Type: image/png")). If Content-Type: header was send, output character encoding conversion will not be performed.
Note that if 'Content-Type: text/*' was send using header(), the sending data is regarded as text, encoding conversion will be performed using character encoding settings.
If you want to output some binary data such as image from PHP script with PHP 4.2.x or earlier, you must set output encoding to "pass" using mb_http_output().
See also ob_start().