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# Pseudo-types used in this documentation

## mixed

`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.

## number

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

## callback

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.

Example 7-11. Callback function examples
    <?php

    // simple callback example
    function foobar() {
        echo "hello world!";
    }
    call_user_function("foobar");

    // method callback examples
    class foo {
      function bar() {
        echo "hello world!";
      }
    }

    $foo = new foo;

    call_user_function(array($foo, "bar")); // object method call

    call_user_function(array("foo", "bar")); // static class method call

    ?>

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