Slimframwork中的singleton和set之间有什么不同？

I've been working with Slim Framework but there is something I cannot understand, the difference between set and singleton functions, by example, if I want to add a user model to my container I can do this:

$app->container->set('user', function(){
     return new User;
});

or this:

$app->container->singleton('user', function(){
    return new User;
});

And It works ok. So I'm wondering what's the point of use one or another. Thanks for your help.

Maybe an example will help

<?php
require 'vendor/autoload.php';

$app = new \Slim\Slim;
$app->container->set('propA', function(){
    static $cnt = 0;
    return ++$cnt;
});

$app->container->singleton('propB', function(){
    static $cnt = 0;
    return ++$cnt;
});

for($i=0; $i<4; $i++) {
    // the function "behind" propA is called every time
    // when propA is accessed
    echo $app->propA, "
";
}
echo "
------
";
for($i=0; $i<4; $i++) {
    // the function "behind" propB is called only once
    // and the stored return value is re-used
    echo $app->propB, "
";
}

prints

1
2
3
4

------
1
1
1
1