   #PHP Manual Types NULL Type Juggling

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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.

   Example 6-11. Callback function examples
   <?php
   // simple callback example
   function my_callback_function() {
       echo 'hello world!';
   }
   call_user_func('my_callback_function');
   // method callback examples
   class MyClass {
       function myCallbackMethod() {
           echo 'Hello World!';
       }
   }
   // static class method call without instantiating an object
   call_user_func(array('MyClass', 'myCallbackMethod'));
   // object method call
   $obj = new MyClass();
   call_user_func(array(&$obj, 'myCallbackMethod'));
   ?>
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