Constants, unlike varaibles, cannot be changed. Meaning once you define it it cannot be changed. For example this varaible $fname= "Aananya" can be changed later if need be. define('Aananya', 22) and define('Birthday', 13) cannot be changes. Constants are always uppercase, name start with aletter or underscore, and case-sensitive. Do not have $ and you cannot put a constant within quotation marks.