How to define a variable in PHP?

How to define a variable in PHP?

define("ABC","Online Training");
echo ABC;//Online Training

?>

Also if I change the value of the variable at later stages what will happen

define("ABC","Online Training");
echo ABC;//Online Training
............
............
............
define("ABC","Online Training Institute");
echo ABC;//Online Training

?>

Even if we change the value it will display the previously set value only