Hello, I'm working on a payment gateway plugin for woocommerce,
the problem is the return url page always shows '1' and doesn't execute
the callback function, has anyone experience with developing a payment
plugin for woocommerce?

How Are u getting the returned url?

mj:
How Are u getting the returned url?

WC()->api_request_url( 'my_payment_gateway' );

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Sanuzi:
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You need to add a Listener.

class is class WC_Gateway_PayNow extends WC_Payment_Gateway {}

// Payment listener/API hook
add_action does this add_action( 'woocommerce_api_wc_gateway_paynow', array( $this, 'check_ipn_response' ) );

function check_ipn_response() {
//handle the responses here returned.
}

mj:
You need to add a Listener.
class is class WC_Gateway_PayNow extends WC_Payment_Gateway {}
// Payment listener/API hook
add_action does this add_action( 'woocommerce_api_wc_gateway_paynow', array( $this, 'check_ipn_response' ) );
function check_ipn_response() {
//handle the responses here returned.
}

Thanks, that was in place but still got the error.

Have this in the check_ipn_response() function. supply the order Id.
$orderId = $posted['orderID'];
$order = new WC_Order( (int) $order_id );
$redirect_url = $this->get_return_url( $order );
wp_redirect( $redirect_url );

Try this or paste your code for us to debug.

Thanks.