How is a constant defined in a PHP script?

A constant is defined in a PHP script using the define() function. The define() function takes two arguments: the constant name (a string) and its value. For example, you can define a constant named MY_CONSTANT with a value of 123 using the following syntax: define('MY_CONSTANT', 123);. Once defined, constants cannot be changed or redefined during the execution of the script. They are typically used to represent values that remain constant throughout the script execution, such as configuration settings or mathematical constants. Constants are case-sensitive by default, but you can make them case-insensitive by passing true as the third argument to the define() function. It's important to note that constants do not require a $ symbol like variables do.