Regarding filter hook for Simple Paypal Payment

In “https://ja.wordpress.org/plugins/simple-paypal-payment/”, sample of filter hook is disclosed, i.e.,;
– “// Filter hook
add_filter(‘simple_paypal_payment_charge’, ‘paypal_charge’, 10, 6);
// Function
public function paypal_charge( $data, $payment_data, $email, $amount, $currency, $payname ) {
if ( $payname === “testpay” ) {
// Please write the process to be done when billing succeeds.
}
}”

Although I have tried to paste this filter hook into php file, I got fatal error which is “syntax error, unexpected ‘public’ (T_PUBLIC) …”.

May I have answer regarding where I have to paste it into ?

Please note that I can use not only English but also Japanese in this matter.

Thank you for your continuous cooperation.

The page I need help with: [log in to see the link]