PHP/Date/cal to jd

Using the French Republican calendar

 
<?php
$jd = cal_to_jd(CAL_FRENCH, 8, 13, 11);
$french = cal_from_jd($jd, CAL_FRENCH);
$gregorian = cal_from_jd($jd, CAL_GREGORIAN);
?>

Using the Jewish calendar

 
<?php
$jd = cal_to_jd(CAL_JEWISH, 6, 14, 5761);
$jewish = cal_from_jd($jd, CAL_JEWISH);
$gregorian = cal_from_jd($jd, CAL_GREGORIAN);
?>

Using the Julian calendar

 
<?php
$jd = cal_to_jd(CAL_JULIAN, 2, 29, 1900);
$julian = cal_from_jd($jd, CAL_JULIAN);
$gregorian = cal_from_jd($jd, CAL_GREGORIAN);
?>