days of the week in php code example

Example 1: php week of a date

Things to be aware of when using week numbers with years.

<?php
echo date("YW", strtotime("2011-01-07")); // gives 201101
echo date("YW", strtotime("2011-12-31")); // gives 201152
echo date("YW", strtotime("2011-01-01")); // gives 201152 too
?>

BUT

<?php
echo date("oW", strtotime("2011-01-07")); // gives 201101
echo date("oW", strtotime("2011-12-31")); // gives 201152
echo date("oW", strtotime("2011-01-01")); // gives 201052 (Year is different than previous example)
?>

Reason:
Y is year from the date
o is ISO-8601 year number
W is ISO-8601 week number of year

Conclusion:
if using 'W' for the week number use 'o' for the year.

Example 2: Number of week days between two dates in php

You are given two string (dd-mm-yyyy) representing two date, 
you have to find number of all weekdays present in between given dates.
  
function number_of_days($days, $start, $end) {
	$start = strtotime($start); $end = strtotime($end);
	$w = array(date('w', $start), date('w', $end));
	$x = floor(($end-$start)/604800);
	$sum = 0;
	for ($day = 0;$day < 7;++$day) {
		if ($days & pow(2, $day)) {
			$sum += $x + ($w[0] > $w[1]?$w[0] <= $day || $day <= $w[1] : $w[0] <= $day && $day <= $w[1]);
		}
	}
	return $sum;
}

function getWeeklyDayNumbers($startDate, $endDate) {
	$weekdays = array('monday' => 0, 'tuesday' => 0, 'wednesday' => 0, 'thursday' => 0, 'friday' => 0, 'saturday' => 0, 'sunday' => 0);
	$weekdays['monday'] += number_of_days(0x02, $startDate, $endDate); // MONDAY
	$weekdays['tuesday'] += number_of_days(0x04, $startDate, $endDate); // TUESDAY
	$weekdays['wednesday'] += number_of_days(0x08, $startDate, $endDate); // WEDNESDAY
	$weekdays['thursday'] += number_of_days(0x10, $startDate, $endDate); // THURSDAY
	$weekdays['friday'] += number_of_days(0x20, $startDate, $endDate); // FRIDAY
	$weekdays['saturday'] += number_of_days(0x40, $startDate, $endDate); // SATURDAY
	$weekdays['sunday'] += number_of_days(0x01, $startDate, $endDate); // SUNDAY
	return $weekdays;
}

$weekdays = getWeeklyDayNumbers('01-01-2021', '31-01-2021');
print_r($weekdays);exit;