Const keyword

The const keyword defines a constant.
Constants are much like variables, except for the following differences:
(i) It is define must be inside the class.
(ii) A constant’s value cannot be changed after it is set.
(iii) Constants do not have a dollar sign ($) before them.

Note:-
(i) Inside the class then values of the constants can be get using self keyword.
(ii) accessing the value outside the class you have to use Scope Resolution Operator ::

Example:-

<?php 
class Employee {
const sift = "Office timming is 10am to 6pm.";
const company_name="Company name is TCS.";
function displaysift_timming()
{
echo self::sift;
}
}
echo Employee::company_name;
$emp_obj = new Employee;
$emp_obj->displaysift_timming();
?>