Replacing parameters in string

Dear users,
I have an array like this:

$par['quantity'] = '10';
$par['sender'] = 'Peter';
$par['date'] = '12/12/2012';

And a string like this:
$str = ‘{$sender} sent you {$quantity} objects on {$date}’;

I want to replace every {$variable} with $par[‘variable’]
I know I have to use preg_replace()

How can I do?

Thanks