(PHP 4, PHP 5, PHP 7, PHP 8)
ob_start — Turn on output buffering
$callback = null, int $chunk_size = 0, int $flags = PHP_OUTPUT_HANDLER_STDFLAGS): boolThis function will turn output buffering on. While output buffering is active no output is sent from the script (other than headers), instead the output is stored in an internal buffer.
The contents of this internal buffer may be copied into a string variable using ob_get_contents(). To output what is stored in the internal buffer, use ob_end_flush(). Alternatively, ob_end_clean() will silently discard the buffer contents.
Some web servers (e.g. Apache) change the working directory of a script
when calling the callback function. You can change it back by e.g.
chdir(dirname($_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.
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:
$buffer, int $phase = ?): stringbufferphasePHP_OUTPUT_HANDLER_* constants.
If callback returns false original
input is sent to the browser.
The callback parameter may be bypassed
by passing a null value.
ob_end_clean(), ob_end_flush(),
ob_clean(), ob_flush() and
ob_start() may not be called from a callback
function. If you call them from callback function, the behavior is
undefined. If you would like to delete the contents of a buffer,
return "" (a null string) from callback function.
You can't even call functions using the output buffering functions like
print_r($expression, true) or
highlight_file($filename, true) from a callback
function.
Note:
ob_gzhandler() function exists to facilitate sending gz-encoded data to web browsers that support compressed web pages. ob_gzhandler() determines what type of content encoding the browser will accept and will return its output accordingly.
chunk_size
If the optional parameter chunk_size is passed, the
buffer will be flushed after any output call which causes the buffer's
length to equal or exceed chunk_size. The default
value 0 means that the output function will only be
called when the output buffer is closed.
flags
The flags parameter is a bitmask that controls
the operations that can be performed on the output buffer. The default
is to allow output buffers to be cleaned, flushed and removed, which
can be set explicitly via
PHP_OUTPUT_HANDLER_CLEANABLE |
PHP_OUTPUT_HANDLER_FLUSHABLE |
PHP_OUTPUT_HANDLER_REMOVABLE, or
PHP_OUTPUT_HANDLER_STDFLAGS as shorthand.
Each flag controls access to a set of functions, as described below:
| Constant | Functions |
|---|---|
PHP_OUTPUT_HANDLER_CLEANABLE |
ob_clean(), ob_end_clean(), and ob_get_clean(). |
PHP_OUTPUT_HANDLER_FLUSHABLE |
ob_end_flush(), ob_flush(), and ob_get_flush(). |
PHP_OUTPUT_HANDLER_REMOVABLE |
ob_end_clean(), ob_end_flush(), and ob_get_flush(). |
Returns true on success or false on failure.
Example #1 User defined callback function example
<?php
function callback($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>
Example #2 Creating an unerasable output buffer
<?php
ob_start(null, 0, PHP_OUTPUT_HANDLER_STDFLAGS ^ PHP_OUTPUT_HANDLER_REMOVABLE);
?>