Which of the following statements about constants in PHP is correct?

The statement that constants are case-sensitive by default is accurate. In PHP, when you define a constant, the default behavior is that its name is case-sensitive. This means that if you define a constant as MY_CONSTANT, it would be distinct from my_constant. You would need to use the exact case when referring to it throughout your code. This characteristic helps in maintaining clarity and consistency, especially in applications where naming conventions are crucial.

Constants in PHP are defined using the define() function and are intended to hold a fixed value that cannot change during the script execution. This immutability is useful for values that should remain constant throughout the lifecycle of the application. As for the other statements, constants cannot store variables, and once a constant is defined, you cannot redefine it or change its value, regardless of case sensitivity. These properties underline the function and purpose of constants in programming.