They might be simple input validation filters created using the create_function() function which allowed you to create anonymous functions of sorts in PHP prior

They might be simple input validation filters created using the create_function() function which allowed you to create anonymous functions of sorts in PHP prior to 5.3. You can't assign an anonymous function to a constant. An alternative would have been to include a big switch statement in the param_get_slashed() function but since the same validators are used in a bunch of places it seems cleaner to use anonymous functions and have each function that uses them loop through and call each function passing the parameter named in the array key in as an argument