<?php

echo (getWeekOfMonth( time() ) . "<BR>"); //today
echo (getWeekOfMonth('2015-10-13') . "<BR>");

/**
 * @param $time Either in timestamp or a date format that will be accepted by strtotime()
 * @return day of month (1-based), or bool FALSE on failure
 */
function getWeekOfMonth($time)
{
    // If we have something that is not in timestamp format,
    // convert it, if we can
    if (($tmp = strtotime($time)) !== false) {
        $time = $tmp;
    }

    // Get the day of the month, in numerical format
    $dayOfMonth = date('j', $time);

    // Get the timestamp for the first day of the month
    $firstDayOfMonthTime = $time - ($dayOfMonth - 1) * 60 * 60 * 24;

    // Get the days until week one ends
    $daysUntilWeekOneEnd = 6 - date('w', $firstDayOfMonthTime);

    // Create an array showing the numerical day of hte month when the week ends
    $weekEnd = array();
    $weekEnd[] = $daysUntilWeekOneEnd + 1;
    $weekEnd[] = $daysUntilWeekOneEnd + 8;
    $weekEnd[] = $daysUntilWeekOneEnd + 15;
    $weekEnd[] = $daysUntilWeekOneEnd + 22;
    $weekEnd[] = $daysUntilWeekOneEnd + 29;

    // Loop through the week end numbers until we find the week we are on
    foreach ($weekEnd as $index => $end) {
        if ($dayOfMonth <= $end) {
            // Then return that week plus one, because the array is 0-indexed
            return ($index+1);
        }
    }

    // If we got this far something went wrong. Return false
    return false;
}