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What is a constant in PHP, and how does it differ from a regular variable? How do you safely retrieve values from superglobals like $_GET or $_POST to avoid undefined index errors and potential security issues? What is the difference between class constants and global constants, and how do you define and access a constant within a class? How does the $_SERVER superglobal provide useful information about the current request and server environment? How do enums, introduced in PHP 8.1, provide a more type safe alternative to using class constants for representing a fixed set of related values? What security risks are associated with directly trusting values from superglobals like $_SERVER, and how should applications guard against header spoofing?
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What are PHP superglobals, and what are some of the most commonly used ones?

Beginner

Answer

Superglobals are built in PHP variables that are always accessible from any scope throughout a script, including inside functions and classes, without needing to explicitly declare them as global, and commonly used superglobals include $_GET and $_POST for accessing submitted form or URL data, $_SESSION for accessing session data, $_SERVER for accessing information about the current request and server environment, and $_FILES for accessing uploaded file information.
echo $_SERVER['REQUEST_METHOD'];
$username = $_POST['username'] ?? '';
Real-world example A login form reads the submitted username and password directly from the $_POST superglobal after the form is submitted, without needing to pass that data through any additional function parameters.

Common follow-ups: Why are superglobals accessible everywhere without needing to be declared global first?;What security precautions should be taken when using data from superglobals like $_GET or $_POST?

Security;Form Handling & Validation
All 7 Constants & Superglobals questions