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When using the `define()` function in PHP, attempting to redefine an existing constant will lead to a warning. Constants in PHP are designed to be immutable, meaning once they are defined with a specific name, that name cannot be reused to define a different value. If you try to use `define()` with an already defined constant name, PHP will emit a warning indicating that you're trying to redefine a constant, which is not allowed. This behavior enforces the integrity of the constant's intended use throughout your application.

In contrast, using an uppercase constant name is valid in PHP, as constant naming conventions typically recommend uppercase names, but it does not cause a warning if you use a lowercase name. Defining a constant with a non-constant value or using a string instead of an integer does not trigger warnings either; PHP will handle these cases in a defined manner, and it does not impose restrictions on the type of the constant value.