Julian days begin at noon, while the days of the Gregorian and Julian calendars are usually thought of as beginning at midnight. For the Gregorian and Julian calendars, the function behaves as if the conversion were performed at noon.(PHP 4 >= 4.1.0, PHP 5, PHP 7, PHP 8)
cal_to_jd — Convertit un calendrier en nombre de jours Julien
   cal_to_jd() calcule le nombre de jours Julien
   pour une date, dans le calendrier calendar.
   Les valeurs possibles pour calendar sont
   CAL_GREGORIAN,
   CAL_JULIAN,
   CAL_JEWISH et
   CAL_FRENCH.
  
calendar
       Calendrier à convertir. Un parmi
       CAL_GREGORIAN,
       CAL_JULIAN,
       CAL_JEWISH ou
       CAL_FRENCH.
      
month
       Le mois, sous la forme d'un nombre. L'intervalle valide
       dépend du calendrier calendar
      
day
       Le jour, sous la forme d'un nombre. L'intervalle valide
       dépend du calendrier calendar
      
year
       L'année, sous la forme d'un nombre. L'intervalle valide
       dépend du calendrier calendar
      
Le nombre de jour Julien.
Julian days begin at noon, while the days of the Gregorian and Julian calendars are usually thought of as beginning at midnight. For the Gregorian and Julian calendars, the function behaves as if the conversion were performed at noon.The earliest supported date in the Julian calendar is January 2, 4713 BC. The corresponding Julian day count is 1. PHP treats -4713 as equal to 4713 BC, and does not allow the year 0; this practice is different than the custom of astronomers, who treat -4712 as equal to 4713 BC.