How can special characters like "&" affect PHP form input and processing?

Special characters like "&" can affect PHP form input and processing by potentially breaking the structure of the data being submitted, leading to errors or security vulnerabilities. To prevent this, you can use the htmlspecialchars() function to encode special characters before processing the form input in your PHP code.

// Using htmlspecialchars() to encode special characters in form input
$input_data = htmlspecialchars($_POST['input_field']);