Constants

Constants in PHP are fixed values that do not change during the execution of the script. They can be handy for values that need to be reused often like a website's URL, a company's name, or even a hardcoded database connection string. Unlike variables, once a constant is defined, it cannot be undefined or reassigned. Constants are case-sensitive by default but this can be overridden. They are defined using the define() function or the const keyword. For instance, you can create a constant to hold the value of Pi and call it in your script like this:

define("PI", 3.14);
echo PI; // Outputs: 3.14