What are the best practices for validating user input in PHP to avoid errors like "Undefined index" when accessing form data?

When accessing form data in PHP, it is important to validate user input to avoid errors like "Undefined index." One way to prevent this error is by using the isset() function to check if the form field exists before trying to access its value. This ensures that the script only accesses form data that has been submitted. 

if(isset($_POST['username'])) {
    $username = $_POST['username'];
} else {
    $username = '';
}

// Now $username will either contain the value from the form input or an empty string if the form field was not submitted

Keywords

PHP user input validation form data error handling undefined index

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