## Output Handlers

Output handlers are callables associated with output buffers that are invoked by calling ob_clean(), ob_flush(), ob_end_flush(), ob_get_flush(), ob_end_clean(), ob_get_clean() or during PHP's shutdown process.

> Note: The shutdown process will flush the return value of the handler.

If omitted or `null` when starting the output buffer the internal `"default output handler"` will be used which returns the unmodified contents of the buffer when invoked. Output handlers can be used to return a modified version of the buffer's contents and/or have side-effects (e.g. send headers).

PHP comes with two internal output handlers: `"default output handler"` and `"URL-Rewriter"` (which is integrated into its own output buffer and only up to two instances of it can be started).

The bundled extensions include four additional output handlers: mb_output_handler(), ob_gzhandler(), ob_iconv_handler(), ob_tidyhandler().

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