A constant is an identifier (name) for a simple value. As the name suggests, that value cannot change during the execution of the script (except for magic constants, which aren’t actually constants). A constant is case-sensitive by default. By convention, constant identifiers are always uppercase.
The name of a constant follows the same rules as any label in PHP. A valid constant name starts with a letter or underscore, followed by any number of letters, numbers, or underscores. As a regular expression, it would be expressed thusly: [a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*
You can define a constant by using the define()-function or by using the const keyword outside a class definition as of PHP 5.3.0. Once a constant is defined, it can never be changed or undefined.
define(“CONSTANT”, “Hello world.”);
echo CONSTANT; // outputs “Hello world.”
echo Constant; // outputs “Constant” and issues a notice.
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