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# ob_start

(PHP 4 , PHP 5)

ob_start -- Turn on output buffering

## Description

bool ob_start ( [callback output_callback [, int chunk_size [, bool erase]]] )

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.

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 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 `PHP_OUTPUT_HANDLER_START`, `PHP_OUTPUT_HANDLER_CONT` and `PHP_OUTPUT_HANDLER_END`.

> 注: 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.

> 注: Before PHP 4.3.2 this function did not return `FALSE` in case the passed output_callback can not be executed.

警告
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. The 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.

範例 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(); ?>`

Would produce:

    <html>
    <body>
    <p>It's like comparing oranges to oranges.</p>
    </body>
    </html>

See also ob_get_contents(), ob_end_flush(), ob_end_clean(), ob_implicit_flush(), ob_gzhandler(), ob_iconv_handler() mb_output_handler(), and ob_tidyhandler().

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