How can conditional statements be used in PHP to prevent overwriting session data?

To prevent overwriting session data in PHP, you can use conditional statements to check if a specific session variable already exists before assigning a new value to it. By checking if the session variable is set, you can avoid overwriting any existing data and ensure that each session variable is only set once. 

session_start();

if(!isset($_SESSION['username'])){
    $_SESSION['username'] = 'JohnDoe';
}

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