PHP Functions Essential Reference

define

bool define(string name, mixed value, [integer case_insensitive]) 

name	Name of the constant
value	Value of the constant
case_insensitive	Whether the constant name is case-insensitive

Defines a named constant.

Returns:

TRUE on success; FALSE otherwise

Description:

define() is used to define named constants. Constants are similar to variables, except for the following differences:

• Once defined, a constant’s value cannot be changed.

• Constant names do not need a leading dollar sign ($).

• Once defined, a constant can be accessed regardless of scope. However, constants can only be accessed in a script after they have been defined.

• Only scalar values (strings and numbers) can be represented as constants.

• Like variables, constant names are case-sensitive. ...