What best practices should be followed when constructing HTML forms in PHP to avoid common pitfalls like missing form values?

When constructing HTML forms in PHP, it is important to ensure that all form values are properly validated and sanitized to avoid missing values. One common pitfall is not checking if a form value is set before using it in PHP code, which can result in errors or vulnerabilities. To avoid this, always check if the form value is set using isset() or empty() functions before using it in your PHP code. 

// Example of checking if a form value is set before using it
if(isset($_POST['username'])) {
    $username = $_POST['username'];
    // Process the form value here
} else {
    // Handle the case where the form value is missing
}

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