How can the use of $_POST and $_SESSION variables in PHP code lead to unexpected behavior, as seen in the provided example?

When using $_POST and $_SESSION variables in PHP code, it's important to remember that they are global variables that can be accessed and modified from anywhere in the code. This can lead to unexpected behavior if the variables are not properly sanitized or validated before use. To solve this issue, always validate user input from $_POST variables and avoid storing sensitive information in $_SESSION variables. 

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

// Example of storing non-sensitive information in $_SESSION variables
$_SESSION['user_id'] = $user_id;

Keywords

$_POST $_SESSION unexpected behavior PHP code example

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