PHP constants introduced in detail

Introduction to constants

Constant is an identifier (name) of a simple value. As the name implies, the value cannot be changed during the execution of the script (except for the so-called magic constants, which are not actually constants). Constants are case-sensitive by default. By convention constant identifiers are always uppercase.
Constant names follow the same naming rules as any other PHP tags. Legal constant names begin with a letter or an underscore, followed by any letters, numbers, or underscores. Using regular expression is expressed like this:
[a-zA-Z_\x7f-\xff][a-zA-Z0-9_ \x7f-\xff]* (Note: Here, the letters are a-z, A-Z, and ASCII characters from 127 to 255 (0x7f-0xff))
The same as super globals, the range of constants It's global. Constants can be accessed anywhere in the script regardless of scope. For more information about the function, read Variable Scope in the manual.

Syntax
You can use the define() function to define constants. Once a constant is defined, it cannot be changed or undefined during program execution.
Constants can only contain scalar data (boolean, integer, float and string).
You can get the value of a constant simply by specifying its name. Do not add the $ symbol in front of the constant. If the constant name is dynamic, you can also use the function constant() to read the value of the constant. Use get_defined_constants() to get a list of all defined constants.
Note: Constants and (global) variables are in different name spaces. This means for example TRUE and $TRUE are different.
If an undefined constant is used, PHP assumes that you want the name of the constant itself, as if you were calling it with a string (CONSTANT corresponds to "CONSTANT"). An E_NOTICE level error will be issued. See the manual for why $foo[bar] is wrong (unless you define bar as a constant with define() beforehand). If you just want to check whether a certain constant is defined, use the defined() function.

3. The difference between constants and variables
There is no dollar sign ($) in front of the constant;
Constant can only be defined with the define() function. It cannot be through assignment statements;
Constants can be defined and accessed anywhere regardless of the rules of variable scope;
Once defined, a constant cannot be redefined or undefined;
The value of a constant can only be a scalar .
Constant definitionExample:

<?php 
define("CONSTANT", "Hello world."); 
echo CONSTANT; // outputs "Hello world." 
echo Constant; // outputs "Constant" and issues a notice. 
?>

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