do_action()执行动作

<?php do_action( $tag, $arg... );?>

/**
 * Execute functions hooked on a specific action hook.
 *
 * This function invokes all functions attached to action hook `$tag`. It is
 * possible to create new action hooks by simply calling this function,
 * specifying the name of the new hook using the `$tag` parameter.
 *
 * You can pass extra arguments to the hooks, much like you can with
 * {@see apply_filters()}.
 *
 * @since 1.2.0
 *
 * @global array $wp_filter  Stores all of the filters
 * @global array $wp_actions Increments the amount of times action was triggered.
 *
 * @param string $tag The name of the action to be executed.
 * @param mixed  $arg Optional. Additional arguments which are passed on to the
 *                    functions hooked to the action. Default empty.
 * @return null Will return null if $tag does not exist in $wp_filter array.
 */
function do_action($tag, $arg = '') {
	global $wp_filter, $wp_actions, $merged_filters, $wp_current_filter;
 
	if ( ! isset($wp_actions[$tag]) )
		$wp_actions[$tag] = 1;
	else
		++$wp_actions[$tag];
 
	// Do 'all' actions first
	if ( isset($wp_filter['all']) ) {
		$wp_current_filter[] = $tag;
		$all_args = func_get_args();
		_wp_call_all_hook($all_args);
	}
 
	if ( !isset($wp_filter[$tag]) ) {
		if ( isset($wp_filter['all']) )
			array_pop($wp_current_filter);
		return;
	}
 
	if ( !isset($wp_filter['all']) )
		$wp_current_filter[] = $tag;
 
	$args = array();
	if ( is_array($arg) && 1 == count($arg) && isset($arg[0]) && is_object($arg[0]) ) // array(&$this)
		$args[] =& $arg[0];
	else
		$args[] = $arg;
	for ( $a = 2; $a < func_num_args(); $a++ )
		$args[] = func_get_arg($a);
 
	// Sort
	if ( !isset( $merged_filters[ $tag ] ) ) {
		ksort($wp_filter[$tag]);
		$merged_filters[ $tag ] = true;
	}
 
	reset( $wp_filter[ $tag ] );
 
	do {
		foreach ( (array) current($wp_filter[$tag]) as $the_ )
			if ( !is_null($the_['function']) )
				call_user_func_array($the_['function'], array_slice($args, 0, (int) $the_['accepted_args']));
 
	} while ( next($wp_filter[$tag]) !== false );
 
	array_pop($wp_current_filter);
}

function func(){
    echo '测试';
}
add_action( 'test', 'func' );
do_action( 'test' );