mixed indicates that a parameter may accept multiple (but not necessarily all) types.
Some functions like call_user_func() or usort() accept user defined callback functions as a parameter. Callback functions can not only be simple functions but also object methods including static class methods.
A PHP function is simply passed by its name as a string. You can pass any built-in or user defined function with the exception of array(), echo(), empty(), eval(), exit(), isset(), list(), print() and unset().
A method of an instantiated object is passed as an array containing an object as the element with index 0 and a method name as the element with index 1.
Static class methods can also be passed without instantiating an object of that class by passing the class name instead of an object as the element with index 0.
Apart from common user-defined function, create_function() can be used to create an anonymous callback function.
Example 11-13. Callback function examples
Note: In PHP4, you will have to use a reference to create a callback that points to the actual object, and not a copy of it. For more details, see References Explained.
void in return type means that the return value is useless. void in parameters list means that the function doesn't accept any parameters.