First app error

Please explain to me this code:

$loader = new \Phalcon\Loader();    
$loader->registerDirs(array(
        '../app/controllers/',
        '../app/models/'
))->register();

It's raise syntax error, but when I change to

$loader = new \Phalcon\Loader();    
$loader->registerDirs(array(
        "../app/controllers/",
        "../app/models/"
))->register();

It's ok. why when I use ' ' it shows an error