Week numbers are defined in this same ISO 8601 standard. Each year has 52 or 53 weeks and weeks always start on a Monday. Week number 1 of each year is the first week in a year that has the first Thursday of the year, or in other words, the week containing January 4th.
He talks about some of the date format arguments that use would use when working with the ISO 8601 formatting and how it relates to the calendar year. He points out that the "Y" format specifier is not the same as the "o" - the first being the calendar year while the second relates to the ISO 8601 year.
As conclusion, this article shows that there are two ways representing dates in PHP. In the Gregorian1 calendar with year, month and day (of month), and in the ISO 8601 calendar with year, week and day (of week). The format characters for the two different years are either Y or o and they should not be confused.