Mage::getSingleton('customer/session')->isLoggedIn()

来判断用户是否登录

比如一般magento开发人员会这样用

<?PHP

//get customer login status ?>

<?php $myStatus = Mage::getSingleton('customer/session')->isLoggedIn() ?>

<?php if($myStatus): ?>

<li><a href="/customer/account/index" title="Customer Register">My account</a> |</li>
<li><?php echo $this->getLayout()->getBlock('header')->getWelcome() ?></li>

<?php else: ?>

<li><a href="/customer/account/index" title="Customer Register">My account</a></li>
<li><a href="/customer/account/create" title="Customer Register">Register</a></li>

<?php endif ?>

但其实在magento里面用户登录状态判断函数早已封装好了.
判断用户登陆状态是否登陆的原理是：Magento在Session中检查CustomerID是否已经设置，并且该CustomerID在数据库中是有效的。

在app/code/core/Mage/Customer/Helper/Data.php文件中

/**
     * Check customer is logged in
     *
     * @return bool
     */
    public function isLoggedIn()
    {
        return Mage::getSingleton('customer/session')->isLoggedIn();
    }

在app/code/core/Mage/Customer/Model/Session.php文件中

/**
     * Checking customer login status
     *
     * @return bool
     */
    public function isLoggedIn()
    {
        return (bool)$this->getId() && (bool)$this->checkCustomerId($this->getId());
    }

所以我们可以在全局用

 if ($this->helper('customer')->isLoggedIn()) { 
  // is logon 
 }