# ob_start

(PHP 4, PHP 5)

ob_start — Turn on output buffering

### Description

bool ob_start ([ callback `$output_callback` [, int `$chunk_size` [, bool `$erase` ]]] )

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.

Warning

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.

### Parameters

`output_callback`

  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. If the `output_callback` is not a callable function, this function will return `FALSE`.

  If the callback function has two parameters, the second parameter is filled with a bit-field consisting of `PHP_OUTPUT_HANDLER_START`, `PHP_OUTPUT_HANDLER_CONT` and `PHP_OUTPUT_HANDLER_END`.

  If `output_callback` returns `FALSE` original input is sent to the browser.

  The `output_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.

  > Note: In PHP 4.0.4, ob_gzhandler() was introduced 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 callback function is called on every first newline after `chunk_size` bytes of output. Default value 0 means that the function is called only in the end, other special value 1 sets `chunk_size` to 4096.

`erase`

  If the optional parameter `erase` is set to `FALSE`, the buffer will not be deleted until the script finishes (as of PHP 4.3.0).

### Return Values

Returns `TRUE` on success or `FALSE` on failure.

### ChangeLog

Version | Description
--- | ---
4.3.2 | This function was changed to return `FALSE` in case the passed `output_callback` can not be executed.

### Examples

Example#1 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();
?>`

The above example will output:

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

### See Also

- ob_get_contents()
- ob_end_clean()
- ob_end_flush()
- ob_implicit_flush()
- ob_gzhandler()
- ob_iconv_handler()
- mb_output_handler()
- ob_tidyhandler()