Refactoring closures/anonymous functions in PHP

Is it possible to refactor the anonymous function out of code like this:

function foo($path, $callback) {
    $callback();
}

$app = array('a', 'b', 'c');

foo('vehicle/:id', function() use ($app) {
    echo $app[0];
});

I tried this but it did not echo anything:

function foo($path, $callback) {
    $callback();
}

$vehicleCallback = function() use ($app) {
    echo $app[0];
};

$app = array('a', 'b', 'c');

foo('vehicle/:id', $vehicleCallback);

Other variations gave me syntax errors. I am looking to move the functions into a separate file, if that matters.

This is my end goal

callbacks.php

$cb1 = function () use ($app) {
    // things
};

$cb2 = function () use ($app) {
    // things
};

// ...more callbacks

router.php

require 'callbacks.php';

$app = new \Foo();

// code that might possibly manipulate $app

$app->bar('/some/relative/path/:param1', $cb1);
$app->bar('/another/relative/path', $cb2);

// possibly more code that mutates $app and then more callbacks

You should definitely turn on error_reporting:

Notice: Undefined variable: app in ... on line 6

When you move line:

$app = array('a', 'b', 'c');

at the beginning of your script you'll get result a

EDIT

You can use $app this way:

<?php

function foo($path, $callback, $app) {
    $callback($app);
};

$vehicleCallback = function($app) {
    echo $app[0];
};

$app = array('a', 'b', 'c');

foo('vehicle/:id', $vehicleCallback, $app);

EDIT2

Sample code for your class

<?php

$cb1 = function ($app, $path) {
    // things
    $app->display('cb1 '.$path.'<br />');
};

$cb2 = function ($app, $path) {
    // things
    $app->display('cb2 '.$path);
};


class Foo {
    public function display ($string) {
        echo strtoupper($string);
    }

    public function bar ($path, $closure) {
        $closure($this, $path);
    }
}


$app = new \Foo();

// code that might possibly manipulate $app

$app->bar('/some/relative/path/:param1', $cb1);
$app->bar('/another/relative/path', $cb2);

// possibly more code that mutates $app and then more callbacks