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.
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 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'));
?>
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