Constants are like variables except that once they are defined they cannot be changed or undefined.
A constant is an identifier (name) for a simple value. The value cannot be changed during the script.
A valid constant name starts with a letter or underscore (no $ sign before the constant name).
Note: Unlike variables, constants are automatically global across the entire script.
To create a constant, use the define() function.
The example below creates a constant with a case-sensitive name:
The example below creates a constant with a case-insensitive name:
Constants are automatically global and can be used across the entire script.
The example below uses a constant inside a function, even if it is defined outside the function: