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