PHP Constants

Conversely to variables, constants are meant for defining immutable values. Constants can be accessed for any scope within a script; however, they can only contain scalar values. Constant names, like variables, are case-sensitive; they also follow the same naming requirements, with the exception of the leading $. It is considered best practice to define constants using only upper-case names.

Here’s an example of constants at work:

define('EMAIL', ''); // Valid name
echo EMAIL; // Displays ''
define('USE_XML', true);
if(USE_XML) { } // Evaluates to true
define('1CONSTANT', ’some value’); // Invalid name

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