Constants are identifiers which are used to store values that cannot be changed once initialized.
A constant doesn’t begin with a dollar ($) sign. The convention for constants is, the name of a constant should always be written in uppercase.
We use the function define to create constants. Below example demonstrates creating and using constants in PHP:
$area = PI*10*10; //Let radius be 10
print("<br />Area of circle is: $area");
A video tutorial on declaring and using constants in PHP: