How can the global evaluation of superglobal variables impact PHP application security and maintainability?

Global evaluation of superglobal variables can introduce security vulnerabilities such as injection attacks and data manipulation. To mitigate these risks, it is recommended to sanitize and validate all user input before using it in the application. This practice helps ensure that only expected and safe data is processed, reducing the likelihood of security breaches. 

// Example of sanitizing and validating user input from a POST request
$username = isset($_POST['username']) ? htmlspecialchars($_POST['username']) : '';
$password = isset($_POST['password']) ? htmlspecialchars($_POST['password']) : '';

// Further validation and processing of the input data
// ...

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