// countries array $countries = array("USA", "Canada", "Australia"); // cities array $cities = array( "USA" => array("New York", "Los Angeles", "Chicago"), "Canada" => array("Toronto", "Vancouver", "Montreal"), "Australia" => array("Sydney", "Melbourne", "Brisbane") ); // HTML code for select boxes echo ""; echo "";In this example, the select box for countries is created using a loop through the countries array, and the `onchange` event is used to submit the form when a country is selected. The select box for cities uses the value from the country select box to display the list of cities. The package library used for this example is the standard PHP library. However, there are also external libraries such as jQuery and Bootstrap that can be used for enhanced functionality and styling.