PHP - Constants Types

A constant is a name or an identifier for a simple value. A constant value cannot change during the execution of the script. By default, a constant is case-sensitive. By convention, constant identifiers are always uppercase. A constant name starts with a letter or underscore, followed by any number of letters, numbers, or underscores. If you have defined a constant, it can never be changed or undefined.

To define a constant you have to use define() function and to retrieve the value of a constant, you have to simply specifying its name. Unlike with variables, you do not need to have a constant with a $. You can also use the function constant() to read a constant's value if you wish to obtain the constant's name dynamically.

constant() function

As indicated by the name, this function will return the value of the constant.
This is useful when you want to retrieve value of a constant, but you do not know its name, i.e. It is stored in a variable or returned by a function.

constant() example

<?php
   define("MINSIZE", 50);
   
   echo MINSIZE;
   echo constant("MINSIZE"); // same thing as the previous line
?>

Only scalar data (boolean, integer, float and string) can be contained in constants.

Differences between constants and variables are

• There is no need to write a dollar sign ($) before a constant, where as in Variable one has to write a dollar sign.
• Constants cannot be defined by simple assignment, they may only be defined using the define() function.
• Constants may be defined and accessed anywhere without regard to variable scoping rules.
• Once the Constants have been set, may not be redefined or undefined.

Valid and invalid constant names

// Valid constant names
define("ONE",     "first thing");
define("TWO2",    "second thing");
define("THREE_3", "third thing");

// Invalid constant names
define("2TWO",    "second thing");
define("__THREE__", "third value"); 

PHP Magic constants

PHP provides a large number of predefined constants to any script which it runs.
There are five magical constants that change depending on where they are used. For example, the value of __LINE__ depends on the line that it's used on in your script. These special constants are case-insensitive and are as follows −
A few "magical" PHP constants are given below −

Sr.No	Name & Description
1	__LINE__ The current line number of the file.
2	__FILE__ The full path and filename of the file. If used inside an include,the name of the included file is returned. Since PHP 4.0.2, __FILE__ always contains an absolute path whereas in older versions it contained relative path under some circumstances.
3	__FUNCTION__ The function name. (Added in PHP 4.3.0) As of PHP 5 this constant returns the function name as it was declared (case-sensitive). In PHP 4 its value is always lowercased.
4	__CLASS__ The class name. (Added in PHP 4.3.0) As of PHP 5 this constant returns the class name as it was declared (case-sensitive). In PHP 4 its value is always lowercased.
5	__METHOD__ The class method name. (Added in PHP 5.0.0) The method name is returned as it was declared (case-sensitive).