Define as name its clear that to declare some thing what is that thing its an variable? no its work as a constant once a define keyword use with any word it will never change or overwrite in php
define Syntax is
define('word','value');
data type contains (Boolean,Integers,Float& Strings).
Example:
<?php
//print my constant
define('mycontnt','php-guides');
echo mycontnt;
//print output php-guides
?>
