How do you define a constant in PHP?

In PHP, constants can indeed be defined using the define() function, which is the most common method for creating a constant. This function accepts two parameters: the name of the constant and its value. Once a constant is defined using define(), it cannot be changed or undefined during the script's execution.

On the other hand, the const keyword is also a valid way to define constants but is typically used in a class context. It is important to note that constants defined using const are available within the scope of the class they are defined in, making them less versatile than the global constants created using the define() function.

The constant() function is not used for defining constants, but rather it is used to retrieve the value of a defined constant. This function requires the name of the constant as its parameter and returns the corresponding value.

Lastly, the set_const() function does not exist in PHP, so it is not a valid choice for defining a constant.

In summary, while the define() function is the primary method for constant creation, understanding the const keyword and the usage of the constant() function are also essential for working effectively with constants in PHP.