We use define() function or const keyword to define a constant. Only scalar data (boolean, integer, float, and string ) can be declared as constants. A resource can be declared as constant but it is not advisable because unexpected result could occur.
Unlike the variables we do not need to prepend $ sign before a constant. To print a constant we just need to mention the name of the constant.
PHP Constant Example:<?php
echo "Output of pi is :".pi."<br/>";
echo "If we would like to print constant within \"\" sign output would be: pi <br/> ";
echo "Since pi is a constant, we need not to use '$' sign before constant";
Output of pi is :3.14
If we would like to print constant within "" sign output would be: pi
Since pi is a constant, we need not to use '$' sign before constant
constab="32"; echo ab; ?>
As in the above example though we want to assign another value to the constant, but the output would be the first value of the variable.
Recommend the tutorial