Write a string produced according to format to the stream resource specified by handle.
Operates as fprintf() but accepts an array of arguments, rather than a variable number of arguments.
See sprintf() for a description of format.
if (!($fp = fopen('date.txt', 'w')))
vfprintf($fp, "%04d-%02d-%02d", array($year, $month, $day));
// will write the formatted ISO date to date.txt