Changing Plugin Load Order

I’m writing a plugin, bar, which is dependent on another plugin, foo, being activated, and I need the functions from foo to be available when bar loads. Normally plugins are loaded in alphabetical order based on the directory name, so bar is loaded before foo. If I rename the bar directory to zbar then it gets loaded last and works fine, but I’m looking for a more elegant and correct solution.

I’ve followed jsdalton’s method of altering the active_plugins option, and it is reordering the array to place bar at the end, but bar still can’t access foo‘s functions when it instantiates. I’ve read through the relevant core code — basically wp-settings.php and wp_get_active_and_valid_plugins() in wp-includesload.php — and it seems like plugins should be loaded in the order they’re indexed inside active_plugins, so I can’t figure out what’s going wrong.

This is on a MultiSite installation. bar is a class and foo is procedural.

Here’s a stripped-down version of foo/foo.php

function foo()
{
    // stuff
}

And here’s a stripped-down version of bar/bar.php

class bar
{
    public function __construct()
    {
        // ...

        $active_plugins = get_option('active_plugins');
        print_r($active_plugins);

        if( function_exists('foo') )
            wp_die("foo exists");
        else
            wp_die("foo doesn't exist yet");
        }
    }
}

function this_plugin_last() 
{
    $wp_path_to_this_file = preg_replace('/(.*)plugins/(.*)$/', WP_PLUGIN_DIR."/$2", __FILE__);
    $this_plugin = plugin_basename(trim($wp_path_to_this_file));
    $active_plugins = get_option('active_plugins');
    $this_plugin_key = array_search($this_plugin, $active_plugins);

    if ($this_plugin_key) 
    {
        array_splice($active_plugins, $this_plugin_key, 1);
        array_push($active_plugins, $this_plugin);
        update_option('active_plugins', $active_plugins);
    }
}
add_action("plugins_loaded", "this_plugin_last");

$bar = new bar();

This is the output from bar‘s constructor:

Array
(
    [0] => foo/foo.php
    [1] => bar/bar.php
)

foo doesn't exist yet