in_array method on volt

You have to add it in your Service defintion:

$di->set('view', function() use ($di) {
    $view = new \Phalcon\Mvc\View();
    $view->setViewsDir(__DIR__ . '/views/');
    $view->setLayoutsDir('/_layouts/');
    $view->setLayout('default');

    $view->registerEngines(['.phtml' => function($view, $di) {
        $volt = new \Phalcon\Mvc\View\Engine\Volt($view, $di);
        $options = [
            'compiledPath' => $di->getConfig()->site->path->cache . 'volt/backend/',
            'compiledExtension' => '.php',
            'compileAlways' => $di->getConfig()->debug,
        ];
        $volt->setOptions($options);

        // Extended functions
        $compiler = $volt->getCompiler();
        $compiler->addFunction('in_array', 'in_array');
        $compiler->addFunction('is_dir', 'is_dir');
        $compiler->addFunction('getimagesize', 'getimagesize');
        return $volt;
    }]);

    return $view;
});

Here is a link with all built in Volt functions. https://docs.phalcon.io/uk/latest/reference/volt.html#functions

To add other functions or create your own refer to the code above.