What PHP function can be used to determine the current calendar week?

To determine the current calendar week in PHP, you can use the `date` function along with the 'W' format character, which represents the ISO-8601 week number of the year. This function will return the current week number based on the ISO-8601 standard, where the week containing January 4th is considered the first week of the year.

$currentWeek = date('W');
echo "The current calendar week is: " . $currentWeek;