Why can't php access external arrays

PHP, as a very popular server-side scripting language, is usually used to develop dynamic websites and web applications. When developers use PHP, they often need to access external arrays to obtain and process data. However, some developers encounter a problem where external arrays cannot be accessed. So, why can't PHP access external arrays?

First, we need to understand how PHP works. PHP is an interpreted language, and the way it works is that PHP code is interpreted into instructions that the machine can understand and then executed line by line. During execution, PHP uses an internal array to store all variables and values. This array is called a SuperGlobal Array.

Super global array is a very important concept in PHP, which contains all variables and data of the PHP program. When a PHP program executes, the superglobal array is automatically populated with the required variables and data. A superglobal array is a special array that can be used to share data throughout a PHP application. Commonly used superglobal arrays include $_GET, $_POST, $_COOKIE and $_SESSION, etc.

However, due to security reasons, PHP does not allow access to external arrays. Code that accesses external arrays can lead to security vulnerabilities because it allows an attacker to tamper with data on the server or execute malicious code by constructing a specific URL. For example, let's say you're using the following code:

$user = $_GET['user'];
$pwd = $_GET['pwd'];

if ($user == 'admin' && $pwd == 'password') {
    echo 'Welcome admin!';
}

If an attacker sets the URL to www.example.com/login.php?user=admin&pwd=password, then they can easily gain unauthorized access to the system and execute malicious code that may harm the system. This is because PHP cannot distinguish which arrays come from legal sources and which arrays come from illegal sources.

To prevent this from happening, PHP uses a mechanism called "safe mode", which restricts the ability to access external arrays. By default, safe mode is enabled and does not allow access to external arrays. Developers can disable safe mode by modifying the PHP configuration file, but this is not recommended as it can lead to security risks.

In addition, PHP also provides some alternatives to obtain and process external data, such as using super-global arrays such as $_POST, $_GET and $_REQUEST, and using the cURL library to obtain remote data. These functions and libraries are considered safe because they have certain security checking mechanisms.

In summary, PHP cannot access external arrays for security reasons. Although developers cannot directly access external arrays, they can use other methods provided by PHP to obtain and process data to ensure the security of the system.

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