To get a date for a specified year and week number, PHP can be implemented using the date() and strtotime() functions in combination. For example, Monday of the 18th week of 2024 can be obtained through the "2024-W18-1" format, and the output is 2024-04-29; if it needs to start with Sunday, you can add 6 days on a Monday basis, such as Sunday of the 18th week of 2024 is 2024-05-05; note that the ISO weekly standard starts on Monday as the weekly starting point, and there may be a New Year's Eve week at the beginning and end of the year. For example, Monday of the 1st week of 2020 is actually 2019-12-30. Therefore, when dealing with boundary situations, you should judge whether to use ISO standards or custom logic based on business needs.

Sometimes we need to obtain the corresponding date based on the given number of weeks and years, such as finding out what day the first day of the 18th week of a certain year is. PHP provides some functions to deal with this type of problem. Although it doesn't seem complicated, it is still prone to errors in details.

Quick fetch using date() and strtotime()

The easiest way is to use PHP's date() and strtotime() functions to combine. For example, if you want to find the start date of a certain week of a certain year (usually Monday), you can write it like this:

 $year = 2024;
$week = 18;

$date = date("Ymd", strtotime("{$year}-W{$week}-1")); // The -1 at the end represents the Monday of the week echo $date; // Output: 2024-04-29

The format "Y-Wn" here is the ISO weekly numbering standard, -1 indicates the Monday of that week. This method is sufficient in most cases.

Pay attention to the weekly starting differences between different systems

Some systems have a week that starts on Sunday, while the ISO standard believes that a week begins on Monday. If your needs are not starting on Monday, you need to adjust manually.

For example, if you want to understand the "first day" of a certain week as a Sunday, you can do it like this:

• Calculate the date of the Monday of the week first
• Then subtract the day and get the last Sunday
• Or add 6 days to get this Sunday

For example:

 $monday = new DateTime("2024-W18-1");
$sunday = clone $monday;
$sunday->modify(' 6 days');
echo $sunday->format('Ym-d'); // Output 2024-05-05

This step seems to be a minor change, but if you are not careful, it is easy to cause the front and back end time logic to not match.

Handling Boundary Situation: Weeks at the beginning and end of the year

Weeks at the beginning and end of the year sometimes cross the New Year, for example, the first week of 2020 actually includes the days at the end of 2019. Using W parameters may be a bit trampled at this time, so you need to pay special attention to whether the results are in line with your business logic.

Let me give you a practical example:

 // I want to find the Monday echo date("Ymd", strtotime("2020-W01-1")); // Output 2019-12-30

You will find that the date you are returning is the 2019 date, because the ISO weekly rules require at least 4 days to be considered week 1 in the New Year. So this week actually starts on December 30, 2019.

If you want to ignore ISO rules and divide the number of weeks by natural months, you may have to write a logical judgment yourself.

Basically that's it. After mastering these key points, it is not difficult to use PHP to obtain the corresponding date of the week.

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