What does using `define()` without third argument do in PHP?

Using define() without the third argument in PHP creates a case-sensitive constant. By default, constants defined using define() are case-sensitive unless you explicitly set the third argument to true, which would then create a case-insensitive constant. This means that if you define a constant called "MY_CONSTANT" and later attempt to access it using "my_constant", it will not be recognized as the same constant due to the case-sensitive nature of its definition. This aspect of the define() function is fundamental in PHP and allows developers to maintain clearer and more intentional naming conventions for their constants depending on their use case, thus avoiding potential bugs that arise from case differences in constant names.

On the other hand, the other options relate to different behaviors of constants or variables in PHP but do not accurately describe the function of define() without the third argument. For instance, creating a variable is not related to the define() function, as variables in PHP are declared with a dollar sign ('$'). Similarly, returning a defined constant value or defining a case-insensitive constant do not pertain to the default behavior of define() when the third argument is omitted.