Php and sql trying to figure out if username exists

This coding asks the user to input their staff ID then the database is searched and their purchase information is displayed in a table (orderID, shippingDate etc.). I want to be able to create and IF statement (quite sure its and IF statement) that makes sure that the staff ID entered exists in the database and if it does continues to the below code otherwise it displays no table just writing saying the the staff id does not exist. The html file is not included as it is not needed. This coding asks the user to input their staff