mb_output_handler Callback function converts character encoding in output buffer stringmb_output_handler stringcontents intstatus 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 hanlder adds charset HTTP header when following conditions are met: Does not set Content-Type by header() Default MIME type begins with text/ http_output setting 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, you must set output encoding to "pass" using mb_http_output. See also ob_start.