Constants in php

• Constant can be defined by using the define() function or by using the const keyword outside the class definition as of php 5.3.0.
• Constant is different from variables in php. To access variable you must use $ prepend but to access constant you should not prepend with a $. With function constant() constant value can also be retrieved.
  
• Constants can't be redefined or undefined once they have been set.
• Unlike variables constants value is not locale. You can access from anywhere within the script.
• A simple example is below.

<?php
define("myCONSTANT", "This is a constant.");
echo myCONSTANT;
?>

which will print output,
This is a constant.

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