Analysis of the functions and usage of PHP predefined variables

The role and usage of PHP predefined variables

In PHP, predefined variables refer to a set of variables automatically defined in the script, which can be used to obtain Runtime information for scripts or receive data from external sources. These variables are available in the global scope and can be used directly without additional declaration. The use of predefined variables can simplify code writing, improve efficiency, and facilitate access to various runtime information.

1. Commonly used PHP predefined variables

1. $_SERVER
   $_SERVER is an array containing information such as request headers, file paths, script names, etc. In PHP are called "super global variables". We can obtain a lot of information related to the server environment through $_SERVER.

For example, the following are some commonly used elements in $_SERVER:

• $_SERVER['HTTP_HOST']: Get the host name of the current request
• $_SERVER['SERVER_ADDR']: Get the IP address of the server
• $_SERVER['REQUEST_METHOD']: Get the method of the current request (GET, POST, etc.)
• $_SERVER['SCRIPT_FILENAME' ]: Get the absolute path of the currently executing script
• $_SERVER['HTTP_USER_AGENT']: Get the user agent information of the current request

Sample code:

echo $ _SERVER['HTTP_HOST']; // Output the host name of the current request
echo $_SERVER['REMOTE_ADDR']; // Output the client's IP address 

2. $_GET and $_POST
   $_GET and $_POST are used to obtain the parameters passed through the GET and POST methods respectively . When passing parameters in the URL, you can use $_GET to obtain them; when submitting data through a form, you can use $_POST to obtain them.

Sample code:

echo $_GET['id']; // Get the value of the id parameter in the URL
echo $_POST['username']; // Get the username submitted in the form 

3. $_SESSION
   $_SESSION is an array variable used to store user session information when the user accesses the website , session information is associated with the user's browser. Functions such as maintaining user login status and storing shopping cart information can be achieved through $_SESSION.

Sample code:

session_start(); // Start session
$_SESSION['username'] = 'Alice'; // Store username in session
echo $_SESSION['username']; // Output the user name

2. How to use PHP predefined variables

1. Get the client IP address
   Through $_SERVER[ 'REMOTE_ADDR'] can obtain the client's IP address, which is usually used to record access logs or perform location tracking and other operations.

Sample code:

$ip = $_SERVER['REMOTE_ADDR'];
echo "The client IP address is:" . $ip;

2. Detection request method
   The current request method can be obtained through $_SERVER['REQUEST_METHOD'], which is usually used to determine whether the user has passed GET or POST method to submit data.

Sample code:

if ($_SERVER['REQUEST_METHOD'] == 'POST') {
    echo "The form has been submitted!";
} else {
    echo "Please submit data through the form!";
}

3. User login verification
   By using $_SESSION to store user information, the user login status can be maintained and verified.

Sample code:

session_start(); // Start session
if (isset($_SESSION['username'])) {
    echo "Welcome back," . $_SESSION['username'] . "!";
} else {
    echo "Please log in first!";
}

Summary: PHP predefined variables are a very convenient tool that can help us obtain various information related to servers, user requests, sessions, etc., simplify code writing, and improve efficiency. By skillfully using PHP predefined variables, we can make our PHP programming more efficient and convenient.

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