Constants

Like any other programming language PHP also allows to define constants. Constants don't change at all. Thus we can define a constant when we know its value will remain same always. Constants can be 'Numeric' or 'String'. 

For example you might have noticed on a shopping store website that there are Page number Links on the bottom of the page, and on each page the number of products displayed is restricted by some number, this can be named as 'ITEMS_PER_PAGE' and is a constant number. Thus this constant is defined as follows