Unable to properly understand vprintf() in PHP

I am unable to understand the following code:

<?php
   $number = 123;
   vprintf("With 2 decimals: %1\$.2f
   <br>With no decimals: %1\$u",array($number));
?>

Browser output:

With 2 decimals: 123.00 
With no decimals: 123

But here there is only one element in array, whereas it has to be two arguments.

Also what is the meaning of %1\$