In what ways can PHP code be optimized to efficiently handle form submissions and calculations while maintaining user input visibility?

To optimize PHP code for handling form submissions and calculations while maintaining user input visibility, you can utilize server-side validation to ensure data integrity and security, use appropriate data structures and algorithms for calculations, and implement AJAX to update parts of the page without refreshing. Additionally, organizing code into functions and classes can improve readability and maintainability.

// Example of server-side validation
if ($_SERVER["REQUEST_METHOD"] == "POST") {
    $inputValue = $_POST["inputValue"];
    
    // Validate input
    if (empty($inputValue)) {
        $error = "Input value is required.";
    } else {
        // Perform calculations
        $result = $inputValue * 2;
    }
}