用户级别输出缓冲区

用户级输出缓冲区可以从 PHP 代码中实现启动、操作和终止。每个都包含输出缓冲区和关联的输出处理函数。

## 打开输出缓冲
可以通过使用 ob_start() 函数或设置 output_buffering 和 output_handler php.ini 配置来启用输出缓冲。虽然两者都可以创建输出缓冲区，但 ob_start() 更加灵活，因为接受用户定义的函数作为输出处理程序，并且还可以设置对缓冲区允许的操作（冲刷、清空、移除）。通过 ob_start() 开始的缓冲区将从调用该函数的行开始生效，而通过 output_buffering 开始的缓冲区将从脚本的第一行开始缓冲输出。

PHP is also shipped with a built-in "URL-Rewriter" output handler which starts its own output buffer and only allows up to two instances of it running at any time (one for user-level URL-rewriting and one for transparent session id support). These buffers can be started by calling the output_add_rewrite_var() function and/or by enabling the session.use_trans_sid php.ini setting.

The bundled zlib extension has its own output buffer which can be enabled by using the zlib.output_compression php.ini setting.

<div class="callout callout-info mb-3">注意: While "URL-Rewriter" is special in that it only allows up to two instances of it running at any one time, all user-level output buffers use the same underlying buffers used by ob_start() with their functionality implemented by a custom output handler function. As such, all of their functionality can be emulated by userland code.</div>

## 冲刷、访问和清理缓冲区内容
Flushing sends and discards the contents of the active buffer. Output buffers get flushed when the size of the output exceeds the size of the buffer; the script ends or ob_flush(), ob_end_flush() or ob_get_flush() is called.

> **警告** Calling ob_end_flush() or ob_get_flush() will turn off the active buffer.

> **警告** Flushing buffers will flush the return value of the output handler which can differ from the contents of the buffer. For example, using ob_gzhandler() will compress the output and flush the compressed output.

The contents of the active buffer can be retrieved by calling ob_get_contents(), ob_get_clean() or ob_get_flush().

If only the length of the buffer's contents are needed, ob_get_length() or ob_get_status() will return the length of the contents in bytes.

> **警告** Calling ob_get_clean() or ob_get_flush() will turn off the active buffer after returning the its contents.

The contents of the active buffer can be cleaned by calling ob_clean(), ob_end_clean() or ob_get_clean().

> **警告** Calling ob_end_clean() or ob_get_clean() will turn off the active buffer.

## 关闭缓冲区
Output buffers can be turned off by calling ob_end_clean(), ob_end_flush(), ob_get_flush() or ob_get_clean().

<div class="callout callout-danger mb-3">**警告** Output buffers started without the PHP_OUTPUT_HANDLER_REMOVABLE flag cannot be turned off and may generate an E_NOTICE.</div>

Every output buffer that has not been closed by the end of the script or when exit() is called will be flushed and turned off by PHP's shutdown process. The buffers will be flushed and turned off in reverse order of their starting up. The last buffered started will be first, the first buffer started will be last to be flushed and turned off.

<div class="callout callout-danger mb-3">**警告** If flushing of the buffer's contents is not desired, a custom output handler should be used to prevent flushing during shutdown.</div>

## 输出处理程序中抛出异常
If an uncaught exception is thrown in an output handler the program terminates and the handler is invoked by the shutdown process after which the "Uncaught Exception" error message is flushed.

If the uncaught exception is thrown in a handler invoked by ob_flush(), ob_end_flush() or ob_get_flush(), the contents of the buffer are flushed before the error message.

If an uncaught exception is thrown in an output handler during shutdown, the handler is terminated and neither the contents of the buffer nor the error message is flushed.

<div class="callout callout-info mb-3">注意: If a handler throws an exception its PHP_OUTPUT_HANDLER_DISABLED status flag is set.</div>

## 输出处理程序中引发错误
If a non-fatal error is raised in an output handler the program continues execution.

If the non-fatal error is raised in a handler invoked by ob_flush(), ob_end_flush() or ob_get_flush(), the buffer flushes certain data depending on the return value of the handler. If the handler returns false the buffer and the error message are flushed. If the returns anything else the handler return value is flushed but not the error message.

<div class="callout callout-info mb-3">注意: If a handler returns false its PHP_OUTPUT_HANDLER_DISABLED status flag is set.</div>

If a fatal error is raised in an output handler the program terminates and the handler is invoked by the shutdown process after which the error message is flushed.

If the fatal error is raised in a handler invoked by ob_flush(), ob_end_flush() or ob_get_flush(), the contents of the buffers are flushed before the error message.

If a fatal error is raised in an output handler during shutdown the program terminates without flushing the buffer or the error message.

## 输出处理程序中输出
In specific circumstances, output produced in the handler is flushed along with the contents of the buffer. This output is not appended to the buffer and is not part of the string returned by ob_get_flush().

During flush operations (calling ob_flush(), ob_end_flush(), ob_get_flush() and during shutdown) if the return value of a handler is false the contents of the buffer are flushed followed by the output. If the handler is not invoked during shutdown the handler throwing an exception or exit() being called results in the same behavior.

<div class="callout callout-info mb-3">注意: If a handler returns false its PHP_OUTPUT_HANDLER_DISABLED status flag is set.</div>

## 输出处理程序状态 flag
The handler status flags of the buffer's flags bitmask are set every time to the output handler is invoked and are part of the flags returned by ob_get_status(). If the handler successfully executes and does not return false, PHP_OUTPUT_HANDLER_STARTED and PHP_OUTPUT_HANDLER_PROCESSED is set. If the handler returns false or throws an exception while executing, PHP_OUTPUT_HANDLER_STARTED and PHP_OUTPUT_HANDLER_DISABLED is set.

<div class="callout callout-info mb-3">注意: If the PHP_OUTPUT_HANDLER_DISABLED of a handler is set, the handler will not be invoked by calling ob_end_clean(), ob_end_flush(), ob_get_clean(), ob_get_flush() or during PHP's shutdown process. This flag has no effect on when calling ob_clean() or ob_flush().</div>