define("CONSTANT", "Hello world.");

echo CONSTANT;  // Outputs Hello world

Constant means a unchangeable value and in php we are
written in "".

"constant";

And don't need to $ sign.

define(my,60);

echo my;

output will be 60.

define(my,vaneet);

echo my;

output will be vaneet

A Dollar($) Symbol is not necessary for displaying a
constant6 variable,
ex:
define("PAGE_SIZE","100");
echo PAGE_SIZE;
or
<?=PAGE_SIZE?>

define("CONSTANT", "Hello world.");

echo CONSTANT;  // Outputs Hello world


define("CONSTANT", "New Value");

echo CONSTANT;  // Outputs Hello world

Means once you declair the define a constant value that 
will become every time it will not take new value or it 
will not redeclair.if you want change in value then declair 
the variables for that purpose.