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Variables and Constant both carries some defined value but difference comes with their accessibility and value.
Constant can be define as
1. define('CONSTANT_NAME', 'constant_value'); OR
2. const CONSTANT_NAME = 'constant_value';
The second way is faster than the first one.
we can not change or over-write the value of constant throughout the application, therefore we can access the same value throughout the application. Doing so will cause fatal error.
Variable:
But in case of Variable, which can contain values in array, string, integer etc. format can be over-written by the function and their accessibility scope is very rigid sometimes only for a function or class or sub-classes.
constant() function:
now Constant() function: is use to access the value of defined constant ex: constant('CONSTANT_NAME');will return value 'constant_value'; from the above example.