PHP Constants

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]*

Example #1 Valid and invalid constant names

<?php
// Valid constant namesdefine("FOO",     "something");define("FOO2",    "something else");define("FOO_BAR", "something more");
// Invalid constant namesdefine("2FOO",    "something");
// This is valid, but should be avoided:
// PHP may one day provide a magical constant
// that will break your scriptdefine("__FOO__", "something");
?>

Note: For our purposes here, a letter is a-z, A-Z, and the ASCII characters from 127 through 255 (0x7f-0xff).