mixed indica que um parâmetro pode aceitar vários (mas não necessariamente todos) os tipos
gettype(), por exemplo, aceita todos os tipos do PHP, enquanto str_replace() somente aceita strings e arrays.
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 builtin 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.