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(PHP 4, PHP 5)

ob_start — Turn on output buffering


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.


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.



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.


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 . Default value 0 means that the function is called only in the end, other special value 1 sets chunk_size to 4096.


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.


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


Example#1 User defined callback function example


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


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



The above example will output:

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