The PHP constant defaults to case sensitive.Traditionally, constant identifiers are always capitalized.

The PHP constant name follows the same naming rules as any other PHP tag.Legal constant names start with a letter or an underscore followed by any letter, number, or underscore.Expressed in a regular expression: [a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*.

<?php
//  Legal Constant Name
define("FOO",     "something");
define("FOO2",    "something else");
define("FOO_BAR", "something more");
//  Illegal constant name
define("2FOO",    "something");
//  The following definitions are legal but should be avoided: ( Custom constants should not be __ Start )
//  Maybe in the future 1 day PHP Will define 1 individual __FOO__ Magic Constant of
//  This will conflict with your code
define("__FOO__", "something");
?>

Summary: 1. Custom Constants *Must be defined with function define() * Value cannot be changed after definition *Use constants directly instead of $s in front like variable 1

2 System Constants: FILE: php program file name LINE: Lines of PHP program files PHP_VERSION: Version number of current parser PHP_OS: The name of the operating system that is executing the current version of PHP _uFILE_uThe name of the script file currently being processed. _uLINE_uCurrent number of lines of script file currently being processed, same as before.

TRUE represents the true value (true). FALSE represents a pseudo value (false).

E_The ERROR constant refers to the most recent error. E_WARNING refers to the most recent warning. E_PARSE is a potential problem with parsing grammar.