When working with PHP, you may need to set an ISO week date on a mutable DateTime. This guide explains the operation, shows what its arguments mean, and uses three separate examples to make the result easier to understand.
date_isodate_set: ISO weeks begin on Monday and can belong to a different week-year than the calendar year.
What Is date_isodate_set in PHP?
Set an ISO week date on a mutable DateTime. ISO weeks begin on Monday and can belong to a different week-year than the calendar year.
Syntax:
date_isodate_set(DateTime $object, int $year, int $week, int $dayOfWeek = 1): DateTime
Parameters and return value: The signature above identifies the arguments and return type where the API has a single call. For topic guides, follow the sequence of calls in the examples. Always distinguish a returned value from an object or variable modified by reference.
Important: Use ISO week-year token o rather than Y when displaying week-based dates.
Example 1: Basic Usage
This example introduces the operation with a short and predictable input.
<?php
$date = new DateTime("2026-01-01", new DateTimeZone("UTC"));
date_isodate_set($date, 2026, 1, 1);
echo $date->format("Y-m-d");
Output:
2025-12-29
The code focuses on the main operation. ISO weeks begin on Monday and can belong to a different week-year than the calendar year.
Example 2: Another Case
Now compare a different input, locale, date, or option to see another part of the behavior.
<?php
$date = new DateTime("2026-01-01", new DateTimeZone("UTC"));
date_isodate_set($date, 2026, 20, 5);
echo $date->format("Y-m-d");
Output:
2026-05-15
What changes? Use ISO week-year token o rather than Y when displaying week-based dates.
Practical Example: Applying It in an Application
Use the following separate example as a starting point for a small application task. Check any extension or environment requirements below before running it.
<?php
$year = 2026;
foreach ([1, 2, 3] as $weekNumber) {
$date = new DateTime("now", new DateTimeZone("UTC"));
date_isodate_set($date, $year, $weekNumber, 1);
echo $date->format("o-\WW-N: Y-m-d") . PHP_EOL;
}
Output:
2026-W01-1: 2025-12-29
2026-W02-1: 2026-01-05
2026-W03-1: 2026-01-12
The practical example demonstrates how to apply this operation to multiple values, a reusable task, or a validated input. ISO weeks begin on Monday and can belong to a different week-year than the calendar year.
Important Behavior and Edge Cases
- Core behavior: ISO weeks begin on Monday and can belong to a different week-year than the calendar year.
- Inputs and results: Use ISO week-year token o rather than Y when displaying week-based dates.
- Alternatives: DateTime::setISODate() is the object-oriented equivalent.
Comparison With Related Functions
| Function or Approach | When to Use It |
|---|---|
date_isodate_set | Set an iso week date on a mutable datetime. |
| Related PHP approach | DateTime::setISODate() is the object-oriented equivalent. |
Common Beginner Mistakes
| Mistake | Correct Approach |
|---|---|
| Using an incorrect input or option | Use ISO week-year token o rather than Y when displaying week-based dates. |
| Confusing this API with a related operation | DateTime::setISODate() is the object-oriented equivalent. |
| Assuming output is identical across environments | Check return values and required extensions or versions. Use explicit locale, timezone or encoding settings when they affect the result. |
Key Takeaways
- date_isodate_set helps you set an ISO week date on a mutable DateTime.
- ISO weeks begin on Monday and can belong to a different week-year than the calendar year.
- Use ISO week-year token o rather than Y when displaying week-based dates.
Use date_isodate_set when you need to set an ISO week date on a mutable DateTime. Start with the first example, compare the second, and adapt the practical example to your inputs. Confirm the return value and environment requirements before adding it to production code.