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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]*
See also the Userland Naming Guide.
Example #5 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 127 through 255 (0x7f-0xff).
Like superglobals, the scope of a constant is global. You can access constants anywhere in your script without regard to scope. For more information on scope, read the manual section on variable scope.