When output_callback is called, it will receive the contents of the output buffer as its parameter and is expected to return a new output buffer as a result, which will be sent to the browser.
An optional output_callback function may be specified. This function takes a string as a parameter and should return a string. The function will be called when ob_end_flush() is called, or when the output buffer is flushed to the browser at the end of the request. When output_callback is called, it will receive the contents of the output buffer as its parameter and is expected to return a new output buffer as a result, which will be sent to the browser.