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# Pseudo-types used in this documentation

## mixed

`mixed` indicates that a parameter may accept multiple (but not necessarily all) types.

gettype() for example will accept all PHP types, while str_replace() will accept strings and arrays.

## number

`number` indicates that a parameter can be either integer or float.

## callback

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.

Apart from common user-defined function, create_function() can be used to create an anonymous callback function.

Example 11-13. Callback function examples

`<?php
// An example callback function
function my_callback_function() {
echo 'hello world!';
}
// An example callback method
class MyClass {
function myCallbackMethod() {
echo 'Hello World!';
}
}
// Type 1: Simple callback
call_user_func('my_callback_function');
// Type 2: Static class method call
call_user_func(array('MyClass', 'myCallbackMethod'));
// Type 3: Object method call
$obj = new MyClass();
call_user_func(array($obj, 'myCallbackMethod'));
?>`

> 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

`void` in return type means that the return value is useless. `void` in parameters list means that the function doesn't accept any parameters.

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