In PHP, a constant is a value that cannot be changed after it has been defined. Constants are useful for storing values that are used repeatedly throughout a program, such as configuration parameters or magic numbers.
To define a constant in PHP, you can use the define() function. For example:
define('PI', 3.14159);
echo PI; // Outputs 3.14159
Alternatively, you can use the const keyword to define a constant. For example:
const PI = 3.14159;
echo PI; // Outputs 3.14159
Once a constant has been defined, you cannot change its value or undefine it.
You can use the defined() function to check if a constant has been defined. For example:
if (defined('PI')) {
echo 'The PI constant is defined';
}