What are some common mistakes made by PHP beginners when trying to perform calculations and display results in a web application?

One common mistake made by PHP beginners when trying to perform calculations and display results in a web application is not properly handling user input validation. It is important to sanitize and validate user input to prevent security vulnerabilities and ensure accurate calculations. Using PHP functions like `filter_var()` can help sanitize user input before performing calculations.

// Example of sanitizing and validating user input before performing calculations
$userInput = $_POST['user_input'];

// Sanitize and validate user input
$filteredInput = filter_var($userInput, FILTER_SANITIZE_NUMBER_FLOAT);

// Perform calculations with sanitized input
$result = $filteredInput * 2;

echo "Result: " . $result;