插件式的结构是一个很不错的结构，整个wordpress就是一个很不错的插件结构。
归跟到底，整个wordpress中的代码都是do_action,apply_fiter，在插件中是用add_action,和add_fiter来注册插件代码。
所以，实现一个插件结构的程序，实现这几个函数就可以了。
首先定义一个插件的目录，本段代码在plugin.php所在目录的plugins目录中，扫描其中所有的php文件，执行其中的代码完成部分插件代码的注册。
如果文件夹中包含目录，那么进入该目录，查找与该目录同名的php文件，包含执行。这样就完成了所有的插件代码注册。

<?php if(!defined('PLUGIN')) {
define('PLUGIN', true);

class plugin {

public $actions;
public $filters;

private $plugin_dir;

private static $instance;

public static function getInstance() {
if(self::$instance == NULL) {
self::$instance = new plugin();
}
return self::$instance;
}

private function __construct() {
$this->actions = $this->filters = array();
$this->plugin_dir = dirname(__FILE__) . "/plugins/";
}

public function init(){

$dir_handle = opendir($this->plugin_dir);
while( $m = readdir($dir_handle)) {
if($m == "." || $m == "..") {
continue;
} else {
$n = $this->plugin_dir . $m;
if(is_file($n)) {
$f = $n;
}else{
$f = $n . "/" . $m . ".php";
}
include $f;
}
}
closedir($dir_handle);
}

public function do_action($action,$params = array()) {
$a = $this->actions[$action];
$c = $a;
if(is_callable($c)){
call_user_func_array($c, $params);
}
}

public function add_action($action,$callable) {
$this->actions[$action] = $callable;
}

public function add_filter($filter,$callable) {
$this->filters[$filter] = $callable;
}

public function apply_filter($filter,$params = array()) {
$c = $this->filters[$filter];
if(is_callable($c)){
call_user_func_array($c, $params);
}
}


}

}

一段包含了插件结果的代码：

<?php
include 'plugin.php';

global $pl;

$pl = plugin::getInstance();

$pl->init();

$pl->do_action("hello", array(0,1,2,3));

$pl->do_action("world", array(0,1,2,3));


echo "<p>___________________________________</p>";

$n = array();

$pl->apply_filter('hello',array(&$n));

echo "<p>__________________________</p>";

var_dump($n);