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). Constants are case-sensitive. By convention, constant identifiers are always uppercase.
Prior to PHP 8.0.0, constants defined using the define() function may be case-insensitive.
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:
See also the Userland Naming Guide.
Example #1 Valid and invalid constant names
// Valid constant names
define("FOO2", "something else");
define("FOO_BAR", "something more");
// Invalid constant names
// This is valid, but should be avoided:
// PHP may one day provide a magical constant
// that will break your script
Note: For our purposes here, a letter is a-z, A-Z, and the ASCII characters from 128 through 255 (0x80-0xff).
Like superglobals, the scope of a constant is global. Constants can be accessed from anywhere in a script without regard to scope. For more information on scope, read the manual section on variable scope.
Note: As of PHP 7.1.0, class constant may declare a visibility of protected or private, making them only available in the hierarchical scope of the class in which it is defined.