PHP: Constants

In PHP, constants are names (identifiers) defined for simple values. Once defined, the value of a constant does not change throughout the program.

Contrast to variables, PHP constants DO NOT start with a $ (dollar) sign. But as with variables, their names also begin either with an alphabet (a-z,A-Z) or with an underscore (_). Constant names may contain alphabets (a-z,A-Z), numbers (0-9) and underscores (_) but they cannot start with a number and cannot contain any other special characters beside the underscore (_). By convention, constant names are typed in all caps. PHP constants are case-sensitive by default.

Values to constants in PHP are assigned using the define() function. Below we assign the value 3.141 to the constant PI 

				
					define('PI', 3.141);

and after defining it, we can use it throughout the program 

				
				echo PI; // prints 3.141
				echo 'Circumference of a circle of radius 5 is', ' ', 2*PI*5; // 2πr

PHP Constants Cannot be Reassigned/Redefined

Once a constant is defined, we cannot reassign it to some other value with an = operator as we do to variables. The below code will produce an error as we try to reassign PI to a new value 3.141592 with an = as we do to variables 

					
						<?php 
							define('PI', 3.141);
							PI = 3.141592;
							echo PI; 
						?>

PHP constants cannot be redefined either (unless you have runkit extension installed). It will always take the first value it is assigned to. Below we redefine PI to a new value 3.141592. But upon printing PI, we find that the value is that of the first value assigned to it 

					
						<?php 
							define('PI', 3.141);
							define('PI', 3.141592);
							echo PI; // 3.141
						?>

Changing Case Sensitivity

The define() function also has a third parameter (optional) for case-insensitivity, which is set to false by default. If we set it to true, we can use the constant in whatever letter case we please. Here we define a constant PI, set the define() function's case-insentivity parameter (the third parameter) to true, and use it in varied cases 

					
						<?php 
							define('PI', 3.141, true);
							echo pi; // 3.141
							echo Pi; // 3.141
							echo pI; // 3.141
						?>

The const Keyword

We can also use the language construct const to define a constant 

					
						<?php 
							const PI = 3.141;
							echo PI; // 3.141
						?>

But const defines constants at compile time, and so defining them conditonally (like inside if .. else statements) will not work. And unlike constants defined by the define() function, constants defined via the const construct are always case-sensitive.