Turn on output buffering

# ob_start

(PHP 4, PHP 5, PHP 7, PHP 8)

ob_start — Turn on output buffering

### Description

ob_start(?callable `$callback` = `null`, int `$chunk_size` = 0, int `$flags` = `PHP_OUTPUT_HANDLER_STDFLAGS`): bool

This function will turn output buffering on. While output buffering is active no output is sent from the script, instead the output is stored in an internal buffer. See What Output Is Buffered? on exactly what output is affected.

Output buffers are stackable, that is, ob_start() may be called while another buffer is active. If multiple output buffers are active, output is being filtered sequentially through each of them in nesting order. See Nesting Output Buffers for more details.

See User-Level Output Buffers for a detailed description of output buffers.

### Parameters

`callback`

  An optional `callback` callable may be specified. It can also be bypassed by passing `null`.

  `callback` is invoked when the output buffer is flushed (sent), cleaned, or when the output buffer is flushed at the end of the script.

  The signature of the `callback` is as follows:

  handler(string `$buffer`, int `$phase` = ?): string
  `buffer`
    Contents of the output buffer.
  `phase`
    Bitmask of `PHP_OUTPUT_HANDLER_*` constants . See Flags Passed To Output Handlers for more details.

  If `callback` returns `false` the contents of the buffer are returned. See Output Handler Return Values for more details.

  Warning

  Calling any of the following functions from within an output handler will result in a fatal error: ob_clean(), ob_end_clean(), ob_end_flush(), ob_flush(), ob_get_clean(), ob_get_flush(), ob_start().

  See Output Handlers and Working With Output Handlers for more details on `callback`s (output handlers).

`chunk_size`

  If the optional parameter `chunk_size` is passed, the buffer will be flushed after any block of code resulting in output that causes the buffer's length to equal or exceed `chunk_size`. The default value `0` means that all output is buffered until the buffer is turned off. See Buffer Size for more details.

`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 the buffer control flags . See Operations Allowed On Buffers for more details.

  Each flag controls access to a set of functions, as described below:

  Constant | Functions
  --- | ---
  `PHP_OUTPUT_HANDLER_CLEANABLE` | ob_clean()
  `PHP_OUTPUT_HANDLER_FLUSHABLE` | ob_flush()
  `PHP_OUTPUT_HANDLER_REMOVABLE` | ob_end_clean(), ob_end_flush(), ob_get_clean(), ob_get_flush()

  > Note: Prior to PHP 8.4.0, the flags parameter could set the output handler status flags as well.

### Return Values

Returns `true` on success or `false` on failure.

### Examples

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);
?>`

### See Also

- ob_get_contents() - Return the contents of the output buffer
- ob_end_clean() - Clean (erase) the contents of the active output buffer and turn it off
- ob_end_flush() - Flush (send) the return value of the active output handler and turn the active output buffer off
- ob_implicit_flush() - Turn implicit flush on/off
- ob_gzhandler() - ob_start callback function to gzip output buffer
- ob_iconv_handler() - Convert character encoding as output buffer handler
- mb_output_handler() - Callback function converts character encoding in output buffer
- ob_tidyhandler() - ob_start callback function to repair the buffer

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