ob_start

Turn on output buffering

&reftitle.description; `bool` `ob_start` `callable` `callback`&null; `int` `chunk_size` 0 `int` `flags` `PHP_OUTPUT_HANDLER_STDFLAGS`

This function will turn output buffering on. While output buffering is active no output is sent from the script (other than headers), instead the output is stored in an internal buffer.

The contents of this internal buffer may be copied into a string variable using `ob_get_contents`. To output what is stored in the internal buffer, use `ob_end_flush`. Alternatively, `ob_end_clean` will silently discard the buffer contents.

Some web servers (e.g. Apache) change the working directory of a script when calling the callback function. You can change it back by e.g. `chdir(dirname($_SERVER['SCRIPT_FILENAME']))` in the callback function.

Output buffers are stackable, that is, you may call `ob_start` while another `ob_start` is active. Just make sure that you call `ob_end_flush` the appropriate number of times. If multiple output callback functions are active, output is being filtered sequentially through each of them in nesting order.

If output buffering is still active when the script ends, PHP outputs the contents automatically.

&reftitle.parameters;

`callback`

- An optional `callback` function may be specified. This function takes a string as a parameter and should return a string. The function will be called when the output buffer is flushed (sent) or cleaned (with `ob_flush`, `ob_clean` or similar function) or when the output buffer is flushed to the browser at the end of the request. When `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. If the `callback` is not a callable function, this function will return &false;. This is the callback signature:

  `string```handler`` `string` `buffer` `int` `phase`

  `buffer`

- Contents of the output buffer.

`phase`

- Bitmask of `PHP_OUTPUT_HANDLER_*` constants.

If `callback` returns &false; original input is sent to the browser.

The `callback` parameter may be bypassed by passing a &null; value.

`ob_end_clean`, `ob_end_flush`, `ob_clean`, `ob_flush` and `ob_start` may not be called from a callback function. If you call them from callback function, the behavior is undefined. If you would like to delete the contents of a buffer, return "" (a null string) from callback function. You can't even call functions using the output buffering functions like `print_r($expression, true)` or `highlight_file($filename, true)` from a callback function.

`ob_gzhandler` function exists to facilitate sending gz-encoded data to web browsers that support compressed web pages. `ob_gzhandler` determines what type of content encoding the browser will accept and will return its output accordingly.

`chunk_size`

- If the optional parameter `chunk_size` is passed, the buffer will be flushed after any output call which causes the buffer's length to equal or exceed `chunk_size`. The default value `0` means that the output function will only be called when the output buffer is closed.

`flags`

- The `flags` parameter is a bitmask that controls the operations that can be performed on the output buffer. The default is to allow output buffers to be cleaned, flushed and removed, which can be set explicitly via `PHP_OUTPUT_HANDLER_CLEANABLE` | `PHP_OUTPUT_HANDLER_FLUSHABLE` | `PHP_OUTPUT_HANDLER_REMOVABLE`, or `PHP_OUTPUT_HANDLER_STDFLAGS` as shorthand.

  Each flag controls access to a set of functions, as described below:

  Constant | Functions
  `PHP_OUTPUT_HANDLER_CLEANABLE` | `ob_clean`, `ob_end_clean`, and `ob_get_clean`.
  `PHP_OUTPUT_HANDLER_FLUSHABLE` | `ob_end_flush`, `ob_flush`, and `ob_get_flush`.
  `PHP_OUTPUT_HANDLER_REMOVABLE` | `ob_end_clean`, `ob_end_flush`, and `ob_get_flush`.

&reftitle.returnvalues;

&return.success;

&reftitle.examples;

### User defined callback function example

    <?php

    function callback($buffer)
    {
      // replace all the apples with oranges
      return (str_replace("apples", "oranges", $buffer));
    }

    ob_start("callback");

    ?>
    <html>
    <body>
    <p>It's like comparing apples to oranges.</p>
    </body>
    </html>
    <?php

    ob_end_flush();

    ?>

&example.outputs;

    <html>
    <body>
    <p>It's like comparing oranges to oranges.</p>
    </body>
    </html>

### Creating an unerasable output buffer

    <?php

    ob_start(null, 0, PHP_OUTPUT_HANDLER_STDFLAGS ^ PHP_OUTPUT_HANDLER_REMOVABLE);

    ?>

&reftitle.seealso;

- `ob_get_contents`
- `ob_end_clean`
- `ob_end_flush`
- `ob_implicit_flush`
- `ob_gzhandler`
- `ob_iconv_handler`
- `mb_output_handler`
- `ob_tidyhandler`