wp_start_template_enhancement_output_buffer(): bool

Source

function wp_start_template_enhancement_output_buffer(): bool {
	if ( ! wp_should_output_buffer_template_for_enhancement() ) {
		return false;
	}

	$started = ob_start(
		'wp_finalize_template_enhancement_output_buffer',
		0, // Unlimited buffer size so that entire output is passed to the filter.
		/*
		 * Instead of the default PHP_OUTPUT_HANDLER_STDFLAGS (cleanable, flushable, and removable) being used for
		 * flags, the PHP_OUTPUT_HANDLER_FLUSHABLE flag must be omitted. If the buffer were flushable, then each time
		 * that ob_flush() is called, a fragment of the output would be sent into the output buffer callback. This
		 * output buffer is intended to capture the entire response for processing, as indicated by the chunk size of 0.
		 * So the buffer does not allow flushing to ensure the entire buffer can be processed, such as for optimizing an
		 * entire HTML document, where markup in the HEAD may need to be adjusted based on markup that appears late in
		 * the BODY.
		 *
		 * If this ends up being problematic, then PHP_OUTPUT_HANDLER_FLUSHABLE could be added to the $flags and the
		 * output buffer callback could check if the phase is PHP_OUTPUT_HANDLER_FLUSH and abort any subsequent
		 * processing while also emitting a _doing_it_wrong().
		 *
		 * The output buffer needs to be removable because WordPress calls wp_ob_end_flush_all() and then calls
		 * wp_cache_close(). If the buffers are not all flushed before wp_cache_close() is closed, then some output buffer
		 * handlers (e.g. for caching plugins) may fail to be able to store the page output in the object cache.
		 * See <https://github.com/WordPress/performance/pull/1317#issuecomment-2271955356>.
		 */
		PHP_OUTPUT_HANDLER_STDFLAGS ^ PHP_OUTPUT_HANDLER_FLUSHABLE
	);

	if ( $started ) {
		/**
		 * Fires when the template enhancement output buffer has started.
		 *
		 * @since 6.9.0
		 */
		do_action( 'wp_template_enhancement_output_buffer_started' );
	}

	return $started;
}

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