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Type Hinting

PHP 5 introduces type hinting. Functions are now able to force parameters to be objects (by specifying the name of the class in the function prototype), interfaces, arrays (since PHP 5.1) or callable (since PHP 5.4). However, if NULL is used as the default parameter value, it will be allowed as an argument for any later call.

If class or interface is specified as type hint then all its children or implementations are allowed too.

Type hints can not be used with scalar types such as int or string. Resources and Traits are not allowed either.

Example #1 Type Hinting examples

<?php
// An example class
class MyClass
{
    
/**
     * A test function
     *
     * First parameter must be an object of type OtherClass
     */
    
public function test(OtherClass $otherclass) {
        echo 
$otherclass->var;
    }


    
/**
     * Another test function
     *
     * First parameter must be an array
     */
    
public function test_array(array $input_array) {
        
print_r($input_array);
    }
    
    
/**
     * First parameter must be iterator
     */
    
public function test_interface(Traversable $iterator) {
        echo 
get_class($iterator);
    }
    
    
/**
     * First parameter must be callable
     */
    
public function test_callable(callable $callback$data) {
        
call_user_func($callback$data);
    }
}

// Another example class
class OtherClass {
    public 
$var 'Hello World';
}
?>

Failing to satisfy the type hint results in a catchable fatal error.

<?php
// An instance of each class
$myclass = new MyClass;
$otherclass = new OtherClass;

// Fatal Error: Argument 1 must be an object of class OtherClass
$myclass->test('hello');

// Fatal Error: Argument 1 must be an instance of OtherClass
$foo = new stdClass;
$myclass->test($foo);

// Fatal Error: Argument 1 must not be null
$myclass->test(null);

// Works: Prints Hello World
$myclass->test($otherclass);

// Fatal Error: Argument 1 must be an array
$myclass->test_array('a string');

// Works: Prints the array
$myclass->test_array(array('a''b''c'));

// Works: Prints ArrayObject
$myclass->test_interface(new ArrayObject(array()));

// Works: Prints int(1)
$myclass->test_callable('var_dump'1);
?>

Type hinting also works with functions:

<?php
// An example class
class MyClass {
    public 
$var 'Hello World';
}

/**
 * A test function
 *
 * First parameter must be an object of type MyClass
 */
function myFunction(MyClass $foo) {
    echo 
$foo->var;
}

// Works
$myclass = new MyClass;
myFunction($myclass);
?>

Type hinting allowing NULL value:

<?php

/* Accepting NULL value */
function test(stdClass $obj NULL) {

}

test(NULL);
test(new stdClass);

?>

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