php中getcwd()、__DIR__、__FILE__ 的作用和区别详解

<?php
spl_autoload_register('sys_autoload');
function sys_autoload($class){
	$name = str_replace('\',DIRECTORY_SEPARATOR,dirname(__FILE__).'/'.$class.'.class.php');
	if(file_exists($name)){
		require_once($name);
	}
}
var_dump(B::getPath01());
var_dump(B::getPath02());
var_dump(B::getPath03());
./a/B.class.php文件内容如下：
<?php
namespace a;

class B{
	public $b = [];
	protected static $instance = null;
	public static function init($config=[]){
		if(B::$instance === null){
			B::$instance = new self;
		}
		return self::$instance;
	}

	private function __construct($config=[]){

	}

	public static function getPath01(){
		return getcwd().DIRECTORY_SEPARATOR;
	}

	public static function getPath02(){
		return dirname(__FILE__).DIRECTORY_SEPARATOR;
	}
	public static function getPath03(){
		return __DIR__.DIRECTORY_SEPARATOR;
	}
}

string(40) "/Library/WebServer/Documents/arithmetic/" 
string(42) "/Library/WebServer/Documents/arithmetic/a/" 
string(42) "/Library/WebServer/Documents/arithmetic/a/"