Week numbers in PHP

How to get the week number from a date

To get the ISO week number (1-53) of a date represented by a DateTime instance, use intval($dateTime->format('W')). Note that format() returns the week number zero-prefixed (e.g. 05), so you need intval to strip the leading zero.

For a date represented by a Unix timestamp, use idate('W', $timestamp) or strftime('%-V', $timestamp) (deprecated since PHP 8.1).

To get the corresponding four-digit year (e.g. 2025), use idate('o', $timestamp) or strftime('%G', $timestamp) or $dateTime->format('o').

Read more about strftime(), idate() and DateTime::format() in the PHP manual.

How to get the date from a week number

To get the Unix timestamp representing the start of the week (Monday at midnight), use strtotime(sprintf("%4dW%02d", $year, $week)).

$year is a 4-digit year (e.g. 2025), and $week is an ISO week number (1-53). The sprintf() call generates a week string in ISO 8601 format, e.g. "2025W01".

To get the start of the week as a DateTime object, use $date = new DateTime('midnight'); $date->setISODate($year, $week);

Read more about strtotime() in the PHP manual.

How to get the number of weeks in a year

To get the number of ISO weeks (i.e. the number of the last week) in a year, use idate('W', mktime(0, 0, 0, 12, 28, $year)).

This is based on the fact that the last week of the year always includes 28 December.

Read more

Learn more about week numbers and the ISO week numbering scheme in this little primer.