PHP 4, PHP 5, PHP 7, PHP 8
ob_start - Turn on output buffering

     [callable$callback = null],
     [int$chunk_size = 0],
): bool

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.

If output buffering is still active when the script ends, PHP outputs the contents automatically.



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:

handler( string$buffer, [int$phase] ): string

Contents of the output buffer.


Bitmask of PHP_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.


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.


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.


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.

Return Values

Returns true on success or false on failure.

Related Functions

Example of ob_start

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Function ob_start:

Output Buffering Control Functions

Most used PHP functions