gateway-settings.php

<?php 

function leaky_paywall_2checkout_gateway_settings( $settings ) {

	if ( !in_array( '2checkout', $settings['payment_gateway'] ) ) {
		return;
	}
	?>
	<table class="form-table">
		<tr>
	        <th><?php _e( '2Checkout Test Seller ID', 'issuem-leaky-paywall' ); ?></th>
	        <td>
	        	<input type="text" id="twocheckout_test_seller_id" class="regular-text" name="twocheckout_test_seller_id" value="<?php echo htmlspecialchars( stripcslashes( $settings['twocheckout_test_seller_id'] ) ); ?>" />
	        	<p class="description"><?php _e( '2Checkout test account number', 'issuem-leaky-paywall' ); ?></p>
	        </td>
	    </tr>
	    
		<tr>
	        <th><?php _e( 'Test Publishable Key', 'issuem-leaky-paywall' ); ?></th>
	        <td>
	        	<input type="text" id="twocheckout_test_publishable" class="regular-text" name="twocheckout_test_publishable" value="<?php echo htmlspecialchars( stripcslashes( $settings['twocheckout_test_publishable'] ) ); ?>" />
	        	<p class="description"><?php _e( 'Test Publishable key', 'issuem-leaky-paywall' ); ?></p>
	        </td>
	    </tr>

	    <tr>
	        <th><?php _e( 'Test Private Key', 'issuem-leaky-paywall' ); ?></th>
	        <td>
	        	<input type="text" id="twocheckout_test_private" class="regular-text" name="twocheckout_test_private" value="<?php echo htmlspecialchars( stripcslashes( $settings['twocheckout_test_private'] ) ); ?>" />
	        	<p class="description"><?php _e( 'Test Private key', 'issuem-leaky-paywall' ); ?></p>
	        </td>
	    </tr>
	 </table>
	<?php 

}
add_action( 'leaky_paywall_payments_settings', 'leaky_paywall_2checkout_gateway_settings', 15, 1 );


function leaky_paywall_update_2checkout_gateway_settings( $settings ) {

	if ( !empty( $_POST['twocheckout_test_seller_id'] ) ) {
		$settings['twocheckout_test_seller_id'] = trim( $_POST['twocheckout_test_seller_id'] );
	}

	if ( !empty( $_POST['twocheckout_test_publishable'] ) ) {
		$settings['twocheckout_test_publishable'] = trim( $_POST['twocheckout_test_publishable'] );
	}

	if ( !empty( $_POST['twocheckout_test_private'] ) ) {
		$settings['twocheckout_test_private'] = trim( $_POST['twocheckout_test_private'] );
	}

	return $settings;

}

add_filter( 'leaky_paywall_update_settings_settings', 'leaky_paywall_update_2checkout_gateway_settings', 10, 1 );