( [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_clean() will silently discard the
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
If output_callback returns FALSE original input
is sent to the browser.
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.
Before PHP 4.3.2 this function did not return FALSE in case the passed
output_callback can not be executed.
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($_SERVER['SCRIPT_FILENAME'])) in the
If the optional parameter chunk_size is passed, the
callback function is called on every first newline after
chunk_size bytes of output.
The output_callback parameter may be bypassed by
passing a NULL value.
If the optional parameter erase is set to FALSE,
the buffer will not be deleted until the script finishes (as of PHP 4.3.0).
Output buffers are stackable, that is, you may call
ob_start() while another
ob_start() is active. Just make
sure that you call ob_end_flush()
the appropriate number of times. If multiple output callback
functions are active, output is being filtered sequentially
through each of them in nesting order.
ob_flush() and ob_start()
may not be called from a callback function. If you call them from
callback function, the behavior is undefined. If you would like to
delete the contents of a buffer, return "" (a null string) from callback
You can't even call functions using the output buffering functions like
print_r($expression, true) or
highlight_file($filename, true) from a callback
Example 1. User defined callback function example
// replace all the apples with oranges
return (str_replace("apples", "oranges", $buffer));
<p>It's like comparing apples to oranges.</p>
<p>It's like comparing oranges to oranges.</p>
See also ob_get_contents(),