胖熊猫的幸福生活

昨天，有个朋友想在人人上做一个链接到他网站上的app。他的网站是使用ThinkPHP框架搭建的。在单独测试人人的app或者单独测试网站程序的时候都没有问题。可是当在他自己的网站中通过人人app访问数据是直接报错了。错误内容大约就是人人app的那个php对象初始化失败，无法加载相应的配置文件，请检查。

通过分析人人app提供的demo可以看出工作流程大体如下：

用户访问第三方app，第三方app从人人取得用户数据，如果没有授权，提示用户授权第三方app访问，之后第三方app就可以从人人获取用户相应的数据了。在第三方app获取人人网的数据时，需要初始化人人网提供的RenrenRestApiService类。通过查看这个类对应的代码可以看到这个类会加载另外两个文件，一个是 ‘HttpRequestService.class.php’，另一个是配置文件 ‘config.inc.php’。

require_once ‘HttpRequestService.class.php’;
require_once ‘config.inc.php’; #Include configure resources

由于报错是在初始化RenrenRestApiService时，所以看了一下RenrenRestApiService的构建函数（如下）

public function __construct(){
    global $config;
    parent::__construct();
    $this->_config = $config;
    if(empty($this->_config->APIURL) || empty($this->_config->APIKey) || empty($this->_config->SecretKey)){
        throw new exception('Invalid API URL or API key or Secret key, please check config.inc.php');
    }
}

在这里会使用到$config。而$config是在’config.inc.php’中定义的，这里使用global标记将其引入到类中。

可是将这个demo中的相关代码放置到ThinkPHP中时初始化RenrenRestApiService却出现了问题。使用var_dump在RenrenRestApiService的构造函数中查看$config变量，提示为空。首先怀疑是由于路径问题，config.inc.php 文件未加载。经过相应的修改后，确定这个文件已经被加载了，而构造函数中的$config还是没有数据。此时，注意到在$config 前有global标记，这个标记是说相关变量引用函数外的同名变量。怀疑应该是这个global 这里出的问题。找到了问题，解决就简单了。首先在在’config.inc.php’中使用$_GLOBALS 的方式定义全局变量$config，之后在RenrenRestApiService的构造函数中使用$_GLOBALS 的方式访问全局变量$config就可以了。

为什么在ThinkPHP中global这个标记不太好用了，暂时不清楚。google了一下，弄清了关于global和$_GLOBALS间的区别，记在这里备查。

以下内容来自 http://stackoverflow.com/questions/3573847/php-global-or-global

Well, you should only use globals in limited circumstances, but to answer your question:

1. global is potentially marginally faster (it will rarely make a difference).
2. $GLOBALS (not $GLOBAL) is more readable, because every time you see it, you know you are accessing/changing a global variable. This can crucial in avoiding nasty bugs.
3. Inside a function, if you want to unset a global variable, you must use unset($GLOBALS['varname']), not global $varname; unset($varname);.

As to points 1 and 2, I’ll quote Sara Golemon here:

What does that mean for your use of the $GLOBALS array? That’s right, the global keyword is technically faster. Now, I want to be really clear about one thing here. The minor speed affordance given by using your globals as localized [compiled variables] needs to be seriously weighed against the maintainability of looking at your code in five years and knowing that $foo came from the global scope. something_using($GLOBALS['foo']); will ALWAYS be clearer to you down the line than global $foo; /* buncha code */ something_using($foo); Don’t be penny-wise and pound foolish..

以下内容来自 http://stackoverflow.com/questions/107693/global-variables-in-php-not-working-as-expected

Global DOES NOT make the variable global. I know it’s tricky

Global says that a local variable will be used as if it was a variable with a higher scope.

E.G :

$var = "test"; // this is accessible in all the rest of the code, even an included one 

function foo2()
{
    global $var;
    echo $var; // this print "test"
    $var = 'test2';
} 

global $var; // this is totally useless, unless this file is included inside a class or function 

function foo()
{
    echo $var; // this print nothing, you are using a local var
    $var = 'test3';
} 

echo $var;  // this will print 'test2'

Note that global vars are rarely a good idea. You can code 99.99999% of the time without them and your code is much easier to maintain if you don’t have fuzzy scopes. Avoid global if you can.

global $foo doesn’t mean “make this variable global, so that everyone can use it”. global $foo means “within the scope of this function, use the  global variable $foo“.

还有一篇文章也不错，比较长，就不全文复制了
http://blog.csdn.net/jiangfeng08/article/details/5416148