Voilà ma version, j'en suis plutôt satisfait :

As PHP doesn't really have a proper alternative to str.format in Python, I decided to implement my very simple own which as most of the basic functionnalitites of the Python's one.

function format($msg, $vars)
{
    $vars = (array)$vars;

    $msg = preg_replace_callback('#\{\}#', function($r){
        static $i = 0;
        return '{'.($i++).'}';
    }, $msg);

    return str_replace(
        array_map(function($k) {
            return '{'.$k.'}';
        }, array_keys($vars)),

        array_values($vars),

        $msg
    );
}

# Samples:

# Hello foo and bar
echo format('Hello {} and {}.', array('foo', 'bar'));

# Hello Mom
echo format('Hello {}', 'Mom');

# Hello foo, bar and foo
echo format('Hello {}, {1} and {0}', array('foo', 'bar'));

# I'm not a fool nor a bar
echo format('I\'m not a {foo} nor a {}', array('foo' => 'fool', 'bar'));

1.    The order doesn't matter,
2.    You can omit the name/number if you want it to simply increment (the first {} matched will be transformed into {0}, etc),
3.    You can name your parameters,
4.    You can mix the three other points.