What is Constants in PHP or PHP Constants

PHP Constants

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-sensitiv. 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.