Get week number in month from date in PHP?

I have an array of random dates (not coming from MySQL). I need to group them by the week as Week1, Week2, and so on upto Week5.

What I have is this:

$dates = array('2015-09-01','2015-09-05','2015-09-06','2015-09-15','2015-09-17');

What I need is a function to get the week number of the month by providing the date.

I know that I can get the weeknumber by doing date('W',strtotime('2015-09-01')); but this week number is the number between year (1-52) but I need the week number of the month only, e.g. in Sep 2015 there are 5 weeks:

• Week1 = 1st to 5th
• Week2 = 6th to 12th
• Week3 = 13th to 19th
• Week4 = 20th to 26th
• Week5 = 27th to 30th

I should be able to get the week Week1 by just providing the date e.g.

$weekNumber = getWeekNumber('2015-09-01') //output 1;
$weekNumber = getWeekNumber('2015-09-17') //output 3;

I think this relationship should be true and come in handy:

Week of the month = Week of the year - Week of the year of first day of month + 1

We also need to make sure that "overlapping" weeks from the previous year are handeled correctly - if January 1st is in week 52 or 53, it should be counted as week 0. In a similar fashion, if a day in December is in the first week of the next year, it should be counted as 53. (Previous versions of this answer failed to do this properly.)

<?php

function weekOfMonth($date) {
    //Get the first day of the month.
    $firstOfMonth = strtotime(date("Y-m-01", $date));
    //Apply above formula.
    return weekOfYear($date) - weekOfYear($firstOfMonth) + 1;
}

function weekOfYear($date) {
    $weekOfYear = intval(date("W", $date));
    if (date('n', $date) == "1" && $weekOfYear > 51) {
        // It's the last week of the previos year.
        return 0;
    }
    else if (date('n', $date) == "12" && $weekOfYear == 1) {
        // It's the first week of the next year.
        return 53;
    }
    else {
        // It's a "normal" week.
        return $weekOfYear;
    }
}

// A few test cases.
echo weekOfMonth(strtotime("2020-04-12")) . " "; // 2
echo weekOfMonth(strtotime("2020-12-31")) . " "; // 5
echo weekOfMonth(strtotime("2020-01-02")) . " "; // 1
echo weekOfMonth(strtotime("2021-01-28")) . " "; // 5
echo weekOfMonth(strtotime("2018-12-31")) . " "; // 6

To get weeks that starts with sunday, simply replace date("W", ...) with strftime("%U", ...).