cal_days_in_month
    (PHP 4 >= 4.1.0, PHP 5)
cal_days_in_month -- Return the number of days in a month for a given year and calendar
Description
int 
cal_days_in_month ( int calendar, int month, int year )
     This function will return the number of days in the
     month of year for
     the specified calendar.
    
     
Example 1. cal_days_in_month() example 
<?php $num = cal_days_in_month(CAL_GREGORIAN, 8, 2003); // 31 echo "There was $num days in August 2003"; ?>
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     See also jdtounix().