Difference between do_action_ref_array() and do_action()

• If you pass an array to do_action_ref_array(), each element’s value of that array will be passed as a separate parameter to the callback. The keys are lost.

• If you pass an array to do_action(), the complete array will be passed as one single parameter to the callback. The keys stay intact.

Let’s look at the differences between do_action() and do_action_ref_array() in a simple plugin:

add_action( 'wp_footer',             'action_diff_test' );
add_action( 'diff_action',           'diff_test_callback', 10, 999 );
add_action( 'diff_action_ref_array', 'diff_test_callback', 10, 999 );

/**
 * call test actions
 */
function action_diff_test()
{
    $args = array (
        'foo'   => 'bar',
        'hello' => 'world',
        0       => 123
    );

    do_action(           'diff_action',           $args );
    do_action_ref_array( 'diff_action_ref_array', $args );
}
/**
 * callback for test actions
 */
function diff_test_callback()
{
    $args = func_get_args();
    print current_filter() . ', ' . func_num_args() . ' arguments';
    ?>
    <pre><?php
    print htmlspecialchars(
        print_r( $args, TRUE ),
        ENT_QUOTES,
        'utf-8',
        FALSE
    );
    ?></pre>
    <?php
}

Result:

diff_action, 1 arguments 
Array
(
    [0] => Array
        (
            [foo] => bar
            [hello] => world
            [0] => 123
        )

)

diff_action_ref_array, 3 arguments 
Array
(
    [0] => bar
    [1] => world
    [2] => 123
)

do_action_ref_array() was introduced 2006 to handle parameters passed by reference better in PHP 4. A review of the function 2011 was closed as wontfix. But the conclusion was not to use for new functions in WordPress anymore, because in PHP 5 you can do that with do_action() too. The problem from 2006 is not a problem nowadays.

Do not use do_action_ref_array() in your custom hooks, but learn how to read it.