This oneliner depends on the fact that PHP treats 0 as false and non-0 as true:
Given the variables $month and $year:
<?php
$daysInMonth = ($month == 2 ? ($year % 4 ? 28 : ($year % 100 ? 29 : ($year % 400 ? 28 : 29))) : (($month - 1) % 7 % 2 ? 30 : 31));
?>
Here is the expanded version for clarity:
<?php
if ($month == 2) // Is it February?
{
   if ($year % 4 != 0) // year not divisible by 4? This February has 28 days
   {
       $daysInMonth = 28;
   }
   else
   {
       if ($year % 100 != 0) // year not divisible by 100? This February has 29 days
       {
            $daysInMonth = 29;
       }
       else
       {
           if ($year % 400 != 0) // year is divisible by 100 but not by 400? This February has 28 days
           {
               $daysInMonth = 28;
           }
           else // year is divisble by 400
           {
               $daysInMonth = 29;
           }
       }
}
else
{
   if (($month - 1) % 7 % 2) == 0)
   {
       $daysInMonth = 30;
   }
   else
   {
       $daysInMonth = 31;
   }
}
?>