Thank you for this bug report. To properly diagnose the problem, we
need a short but complete example script to be able to reproduce
this bug ourselves. 

A proper reproducing script starts with <?php and ends with ?>,
is max. 10-20 lines long and does not require any external 
resources such as databases, etc. If the script requires a 
database to demonstrate the issue, please make sure it creates 
all necessary tables, stored procedures etc.

Please avoid embedding huge scripts into the report.

The following works just fine here:
<?php
date_default_timezone_set("America/Chicago");
$timestamp = mktime(0,0,0,11,1,2014);
$a = strtotime('4 thursdays', $timestamp);
$b = strtotime('first thursday + 3 weeks', $timestamp);

echo date(DateTime::ISO8601, $a), "\n";
echo date(DateTime::ISO8601, $b), "\n";
?>

outputs:
2014-11-27T00:00:00-0600
2014-11-27T00:00:00-0600


This is with PHP 5.4.24

No feedback was provided. The bug is being suspended because
we assume that you are no longer experiencing the problem.
If this is not the case and you are able to provide the
information that was requested earlier, please do so and
change the status of the bug back to "Re-Opened". Thank you.

This still occurs on PHP 5.6.4

<?php
echo date_default_timezone_get() . "\n;
date_default_timezone_set('UTC');
$timestamp = mktime(0,0,0,11,1,2014);
echo strtotime('4 thursdays', $timestamp) . "\n";
echo strtotime('first thursday + 3 weeks', $timestamp) . "\n";
?>

Output:
America/Chicago
1417068000
1417046400

I am unable to change the status back to "unopened" due to "ERROR:
You aren't allowed to change a bug to that state."

sorry, I meant "re-opened"

Interestingly, the issue is timezone related. It doesn't happen for
America/Chicago, but for UTC, for instance[1].

[1] <http://3v4l.org/LLkCE>

Plural of day names are not supported by timelib, so '4 thursdays' will be interpreted as:

'the fourth Thursday, in the "s" - sierra - time zone'

The sierra time zone is UTC-6, and that overlaps with America/Chicago for parts of the year.

I think this should be reclassified as a documentation problem.

The claim on the strtotime page that it will 'Parse about any English textual datetime description into a Unix timestamp' is not very specific, and other time units do support the plural form. It could do with a few more examples, or a pointer to the regexes used in the implementation.

Alternatively I can provide a few-line patch to add the plural of week names, but that will break BC.

However I do not think many people even know about the one letter time zones, except for Zulu, and even fewer will actually use it.

In any case I suggest the test case for this bug be removed from the test suite.

> Plural of day names are not supported by timelib, […]

Spot on!  Thank you for the detailed analysis!

> It could do with a few more examples, or a pointer to the
> regexes used in the implementation.

There is already documentation about the supported date and time
formats[1], and this page is linked from the parameter's
description.  It seems to me that adding the parameter name to the
function description, so that it'll be linked to the parameter's
description, should be sufficient.

> In any case I suggest the test case for this bug be removed from
> the test suite.

Good catch!  <https://github.com/php/php-src/pull/7543>

[1] <https://www.php.net/manual/en/datetime.formats.php>

Support for "thursdays" (with the extra "s") will be part of PHP 8.1.8 and PHP 8.2 once there is a new timelib release with https://github.com/derickr/timelib/commit/cb26187fb6033a80d4bae4883ee6b62eebfd4856 in it.

This makes this issue a duplicate of #51934, and hence I'm closing it.