Gets the number of days in the month.

<?php

/**
 * Returns the number of days in the requested month.
 *
 * @param int month as a number (1-12)
 * @param int the year, leave empty for current
 * @return int the number of days in the month
 */
function days_in_month($month, $year = null) {
  $year  = !empty($year) ? (int) $year : (int) date('Y');
  $month = (int) $month;

  if ($month < 1 || $month > 12) {
   return 0;
  } elseif ($month == 2) {
   if ($year % 400 == 0 || ($year % 4 == 0 && $year % 100 != 0)) {
     return 29;
   }
  }

  $days_in_month = array(
   31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31
  );

  return $days_in_month[$month - 1];
}

// Usage
echo days_in_month("4", 2012); // 30