mixed indicates that a
parameter may accept multiple (but not necesseraly all)
types.
gettype() for example will accept all PHP types,
while str_replace() will accept strings and
arrays.
Some functions like
call_user_function() 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.