User-Level Output Buffers

Turning Output Buffering On

Output buffering can be turned on by using the ob_start function or by setting the output_buffering and output_handler php.ini settings. While both can create output buffers, ob_start is more flexible as it accepts user-defined functions as output handlers and the operations allowed on the buffer (flush, clean, remove) can be set as well. Buffers started with ob_start will be active from the line the function was called, while those started with output_buffering will be buffering output from the first line of the script.

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.

Note: 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.

Flushing, Accessing And Cleaning Buffer Contents

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.

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.

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

Turning Buffers Off

Output buffers can be turned off by calling ob_end_clean, ob_end_flush, ob_get_flush or ob_get_clean.

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.

Exceptions Thrown In Output Handlers

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.

Note: If a handler throws an exception its PHP_OUTPUT_HANDLER_DISABLED status flag is set.

Errors Raised In Output Handlers

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.

Note: If a handler returns false its PHP_OUTPUT_HANDLER_DISABLED status flag is set.

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.

Output In Output Handlers

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.

Note: If a handler returns false its PHP_OUTPUT_HANDLER_DISABLED status flag is set.

Output Handler Status Flags

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.

Note: 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, ob_clean, ob_flush or during PHP's shutdown process. Prior to PHP 8.4.0, this flag had no effect when calling ob_clean or ob_flush.