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 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 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 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. This is the callback signature:
$buffer [, int $phase ] ) If the optional parameter chunk_size is passed, the buffer will be flushed after any output call which causes the buffer's length to equal or exceed chunk_size. The default value 0 means that the output function will only be called when the output buffer is closed.
Prior to PHP 5.4.0, the value 1 was a special case value that set the chunk size to 4096 bytes.
The flags parameter is a bitmask that controls the operations that can be performed on the output buffer. The default is to allow output buffers to be cleaned, flushed and removed, which can be set explicitly via PHP_OUTPUT_HANDLER_CLEANABLE | PHP_OUTPUT_HANDLER_FLUSHABLE | PHP_OUTPUT_HANDLER_REMOVABLE, or PHP_OUTPUT_HANDLER_STDFLAGS as shorthand.
Each flag controls access to a set of functions, as described below:
| Constant | Functions |
|---|---|
PHP_OUTPUT_HANDLER_CLEANABLE | ob_clean(), ob_end_clean(), and ob_get_clean(). |
PHP_OUTPUT_HANDLER_FLUSHABLE | ob_end_flush(), ob_flush(), and ob_get_flush(). |
PHP_OUTPUT_HANDLER_REMOVABLE | ob_end_clean(), ob_end_flush(), and ob_get_flush(). |
Returns TRUE on success or FALSE on failure.
Some web servers (e.g. Apache) change the working directory of a script when calling the callback function. You can change it back by e.g. chdir(dirname($_SERVER['SCRIPT_FILENAME'])) in the callback function.
In case ob_start() is used inside an output buffer callback, this function will no longer issue an E_ERROR but instead an E_RECOVERABLE_ERROR, allowing custom error handlers to catch such errors.
The third parameter of ob_start() changed from a boolean parameter called erase (which, if set to FALSE, would prevent the output buffer from being deleted until the script finished executing) to an integer parameter called flags. Unfortunately, this results in an API compatibility break for code written prior to PHP 5.4.0 that uses the third parameter. See the flags example for an example of how to handle this with code that needs to be compatible with both.
A chunk size of 1 now results in chunks of 1 byte being sent to the output buffer.
This function was changed to return FALSE in case the passed output_callback can not be executed.
<?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>
<?php
if (version_compare(PHP_VERSION, '5.4.0', '>=')) {
ob_start(null, 0, PHP_OUTPUT_HANDLER_STDFLAGS ^
PHP_OUTPUT_HANDLER_REMOVABLE);
} else {
ob_start(null, 0, false);
}
?>
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