What is the purpose of using isset() in PHP form processing?

When processing form data in PHP, it is important to check if a form field has been submitted and has a value before using it in further processing. This helps prevent errors and ensures that the script does not encounter undefined variable warnings. The isset() function in PHP is commonly used to check if a variable is set and is not NULL. 

// Check if the form field 'username' is set and not empty
if(isset($_POST['username'])) {
    $username = $_POST['username'];
    // Further processing of the username variable
} else {
    // Handle the case where the 'username' form field is not set
}

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