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What is a constant in PHP, and how does it differ from a regular variable? What are PHP superglobals, and what are some of the most commonly used ones? 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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How do you safely retrieve values from superglobals like $_GET or $_POST to avoid undefined index errors and potential security issues?

Intermediate

Answer

You should always check whether a specific key exists before accessing it, commonly using the null coalescing operator to provide a safe default value if the key is missing, and you should also validate and sanitize any data coming from these superglobals before using it in your application, since this data originates directly from user input and should never be trusted without verification.
$email = $_POST['email'] ?? '';
if (filter_var($email, FILTER_VALIDATE_EMAIL)) {
    echo 'Valid email';
}
Real-world example A registration form safely retrieves the submitted email address using the null coalescing operator to avoid an error if the field is missing, and validates it using PHP's built in email filter before processing the registration.

Common follow-ups: What is the null coalescing operator and how does it differ from using isset?;What other PHP filter functions exist for validating different types of user input?

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