Checking for existence of constants before defining them

These are not PHP global variables, they are constants that cannot be modified(hence the name).

Trying to redefine them, using a define() will trigger a notice since they are already defined(and consequently keep the initial value assigned). To stay away from notices, you should check if they have been defined() before.

Example:

define('VARIABLE', 'hello');
define('VARIABLE', 'world');
echo VARIABLE;

Results:

Notice: Constant VARIABLE already defined in **your-file** on line xxx
hello

In this case the variable won’t be updated to world.

Example using defined():

define('VARIABLE', 'hello');

if(!defined('VARIABLE')){
  define('VARIABLE', 'world');
}
else{
  echo "VARIABLE HAS ALREADY BEEN DEFINED.<br />";
}
echo VARIABLE;

Results:

VARIABLE HAS ALREADY BEEN DEFINED.
hello

In this case you can specify an action to perform in case that constant has already been defined.