(PHP 4 >= 4.1.0, PHP 5, PHP 7)
cal_to_jd — Converts from a supported calendar to Julian Day Count
$calendar
   , int $month
   , int $day
   , int $year
   )
   cal_to_jd() calculates the Julian day count
   for a date in the specified calendar.
   Supported calendars are
   CAL_GREGORIAN,
   CAL_JULIAN,
   CAL_JEWISH and
   CAL_FRENCH.
  
calendar
       Calendar to convert from, one of 
       CAL_GREGORIAN,
       CAL_JULIAN,
       CAL_JEWISH or
       CAL_FRENCH.
      
month
       The month as a number, the valid range depends 
       on the calendar
      
day
       The day as a number, the valid range depends 
       on the calendar
      
year
       The year as a number, the valid range depends 
       on the calendar
      
A Julian Day number.