add_action()添加动作

功能：

add_action函数用来添加一个函数到指定的动作上，当指定的动作被调用的时候，这个函数也就被调用了。

用法：

参数：

$hook

（字符串）（必须）挂载函数的动作名称。

默认值：None

$function_to_add

（回调函数）（必须）挂载的函数，直接填写字符串形式的函数名即可，如果你要挂载类函数可以参考：https://www.endskin.com/hook-object-function/

默认值：None

$priority

（整数）（可选）动作执行的优先级，数值越小越先被执行。

默认值：10

$accepted_args

（整数）（可选）回调函数接收几个参数。

默认值：1

返回值：

（布尔）始终返回 True.

所在位置：

add_action()函数包含在wp-includes/plugin.php中.

源码：

/**
 * Hooks a function on to a specific action.
 *
 * Actions are the hooks that the WordPress core launches at specific points
 * during execution, or when specific events occur. Plugins can specify that
 * one or more of its PHP functions are executed at these points, using the
 * Action API.
 *
 * @since 1.2.0
 *
 * @param string   $tag             The name of the action to which the $function_to_add is hooked.
 * @param callback $function_to_add The name of the function you wish to be called.
 * @param int      $priority        Optional. Used to specify the order in which the functions
 *                                  associated with a particular action are executed. Default 10.
 *                                  Lower numbers correspond with earlier execution,
 *                                  and functions with the same priority are executed
 *                                  in the order in which they were added to the action.
 * @param int      $accepted_args   Optional. The number of arguments the function accept. Default 1.
 * @return bool Will always return true.
 */
function add_action($tag, $function_to_add, $priority = 10, $accepted_args = 1) {
	return add_filter($tag, $function_to_add, $priority, $accepted_args);
}

示例：

function test_add_action() {
    echo '';
}
add_action( 'wp_head', 'test_add_action' );

相关函数:

do_action()
remove_action()
add_filter()

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