What are common methods used in PHP to validate form data before processing?

Validating form data before processing is crucial to ensure that the data entered by users is correct and safe to use. Common methods used in PHP for form data validation include using built-in functions like `filter_var` to validate input fields such as email addresses, numbers, and URLs. Regular expressions can also be used to check for specific patterns in input data. Additionally, custom validation functions can be created to check for more complex validation rules.

// Example of validating an email address using filter_var
$email = $_POST['email'];

if (filter_var($email, FILTER_VALIDATE_EMAIL)) {
    // Email is valid, proceed with processing
} else {
    // Email is not valid, display an error message to the user
    echo "Invalid email address";
}