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Declaring a constant in PHP can indeed be accomplished using the define() function, which allows for the creation of constant values that cannot be changed after their initial assignment. The define() function is commonly used because it provides flexibility regarding the name and the scope of the constant.

When you use define(), you specify the name of the constant as a string and then assign it a value. For example, define('MY_CONSTANT', 'value'); creates a constant named MY_CONSTANT with the value 'value'. This constant can then be accessed anywhere in the script, and trying to change its value or redeclare it will result in a fatal error.

In contrast, while the const keyword can also be used to declare constants, it is limited to class contexts or global variables and must be declared at compile time. The declare() function is meant for compiler directives, not for creating constants, and the constant() function is actually used to get the value of an existing constant rather than declaring one. These differences illustrate why the correct approach to declaring a constant is through the use of the define() function.