Those two are backed by gettimeofday(2), which is not suitable for measuring time intervals to sub-millisecond precision.

gettimeofday(2) may not be accurate at sub-millisecond level, but with 2 sequential calls the last one will not be lower then the first one.

I think it is caused by the fact that $_SERVER['REQUEST_TIME_FLOAT'] is rounded to milliseconds and microtime(TRUE) is not.

REQUEST_TIME_FLOAT is not rounded.

To make sure, what SAPI are you using for the web side? One can define its own means of determining the request time, and if it does not then PHP falls back to gettimeofday.

https://github.com/php/php-src/blob/PHP-7.4.1/main/SAPI.c#L1083

The SAPI is apache2handler, that one gives indeed msec als precision.

https://github.com/php/php-src/blob/PHP-7.4.1/sapi/apache2handler/sapi_apache2.c#L360

It seems dat usec is also available 

http://apr.apache.org/docs/apr/1.5/group__apr__time.html

> The SAPI is apache2handler, that one gives indeed msec als precision.
> It seems usec is also available

apache2handler on all of my setups is using usec

this is a microtime(TRUE) based runtime measurement from a custom header
0.0245 / 0.0152

the first number is based on $_SERVER['REQUEST_TIME_FLOAT']
the second one is based on microtime(TRUE) as first codeline

--------------------

$cms_stop_microtime = microtime(TRUE);
$runtime_total = round(($cms_stop_microtime - $_SERVER['REQUEST_TIME_FLOAT']), 4);
$tuntime_cms = round(($cms_stop_microtime - $cms_startup_microtime), 4);

--------------------

this configures a additional http header which is also usec
Header set "X-Response-Time" "%D us"

X-Response-Time: D=25726 us

> The SAPI is apache2handler, that one gives indeed msec als precision.
> https://github.com/php/php-src/blob/PHP-7.4.1/sapi/apache2handler/sapi_apache2.c#L360
Also https://apr.apache.org/docs/apr/1.7/group__apr__time.html which shows the time from apr_time_as_msec is truncated to milliseconds.

> apache2handler on all of my setups is using usec

not for REQUEST_TIME_FLOAT, the 2 numbers you give are both based on $cms_stop_microtime, filled with 'microtime(TRUE)', not coming from the sapi module.

with phpinfo(); you can see that the fraction part of REQUEST_TIME_FLOAT is 3 digits, not 6 digits.

What can I do to get this request into 8.0 ?

I have it localy working by changing line 369 of file php-src/sapi/apache2handler/sapi_apache2.c (PHP 8 master branch at github)

return ((double) apr_time_as_msec(ctx->r->request_time)) / 1000.0;

into

return ((double) ctx->r->request_time) / 1000000.0;

and now there is never a negative result anymore in the test script as given in my original post at the top of this page.

also $_SERVER['REQUEST_TIME_FLOAT'] & microtime(true) have now the same precision.

echo $_SERVER['REQUEST_TIME_FLOAT'];
echo '<br>';
echo microtime(true);

It would be great if someone can include this in the PHP 8 branch, or can I send the git diff patch to someone?

Pull request with the change was accepted for the PHP 8 branch at github