What are best practices for handling user input validation and sanitization in PHP forms?

User input validation and sanitization are crucial steps in securing PHP forms against malicious attacks such as SQL injection and cross-site scripting. Best practices include validating input data to ensure it meets expected criteria (e.g., data type, length, format) and sanitizing input to remove any potentially harmful characters.

// Validate and sanitize user input from a form field
$input = $_POST['input_field'];
$clean_input = htmlspecialchars(trim($input));

// Check if input meets specific criteria (e.g., length, data type)
if (strlen($clean_input) < 5) {
    // Handle error - input too short
} else {
    // Process the sanitized input
    // (e.g., save to database, display on webpage)
}