How do you declare a constant in PHP?

In PHP, constants are declared using the define() function. This function takes two parameters: the name of the constant and its value. Once a constant is defined, it cannot be changed or undefined during the script execution, which is one of the key characteristics that distinguish constants from variables.

Using define('NAME', 'value') is the proper way to create a constant because it ensures that the value you assign will remain constant throughout the script and is accessible throughout all scopes. Constants are typically written in uppercase to signify their immutable nature, although this is a convention rather than a requirement.

The other options illustrate incorrect ways of declaring constants in PHP. The constant() function, for instance, is used to retrieve the value of a constant by name, rather than declaring one. The syntax var NAME = 'value' is not a valid way to declare constants as it is meant for declaring variables, lessening the distinction between mutable and immutable values. Lastly, set_constant('NAME', 'value') is not a valid PHP function for declaring constants, as there is no such function in the PHP language.

These distinctions clarify why the use of define() is the correct method for declaring constants in PHP.