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The `count()` function in PHP is specifically designed to handle arrays and objects that implement the Countable interface. When used on an array, it returns the total number of elements within that array. If the function is called on an object, it returns the number of properties in the object if that object is Countable. This means that `count()` is not suited for directly working with strings, numbers, or booleans, as it is intended for counting elements or properties, rather than evaluating or performing operations on non-collection types.

Strings would imply measuring length rather than a count of components, which is handled by the `strlen()` function. Numbers and booleans don’t represent collections of items to be counted in the same way, making the use of `count()` inappropriate for these data types as well. Thus, the focus of `count()` on arrays and Countable objects clearly defines its application in PHP programming.