time_sleep_until

At least on my Windows machine, the time_sleep_until function appears to calculate the number of microseconds between now and the sleep-until timestamp, and it appears to use unsigned 32-bit math in this calculation. This roundoff leads to a maximum sleep time of just under 4295 seconds (1 hour, 11 minutes, 35 seconds). To get longer sleep times, while still using time_sleep_until to minimize processor overhead, the following loop may be some help to you:
<?php
$sleepuntil = strtotime("tuesday 3pm");
while (time() < $sleepuntil)
 time_sleep_until($sleepuntil);
// proceed with dated processing
?>

Of course, one could use something like "cron" instead, to avoid the script doing the extended sleep. Also note that time_nanosleep appears to do similar math, but it is somewhat more intuitive that the seconds parameter has an upper limit on what it can be. Still, both functions might report a warning when waking up prematurely due to roundoff.

Not realising that this function existed, I wrote something similar, but it has the additional facility to specify a minimum pause even if the target time has already been reached, for instance in a processor-intensive loop.
It's in seconds rather than microseconds (it's intended for heavy-duty CLI scripts), but that could easily be changed by using microtime(true) and usleep if greater granularity was required.
<?php
 /**
 * Pause processing until the specified time, to avoid hammering a DB or service
 *
 * @param int $target_time Timestamp
 * @param int $min_sleep Always sleep for a minimum number of seconds,
 * even if the target timestamp has already passed.
 * Default 0, meaning only sleep until the target timestamp is reached.
 *
 * @example <code>
 while ( ! $finished )
 {
 $minimum_start_of_next_loop = time() + $min_secs_per_loop;
 
 # DO STUFF THAT MAY OR MAY NOT TAKE VERY LONG
 
 sleep_until( $minimum_start_of_next_loop, $min_pause_between_loops );
 }
 </code>
 */
 function sleep_until($target_time, $min_sleep = 0)
 {
 $time_now = time();
 
 $time_to_target = $target_time - $time_now;
 
 // If we've already reached the target time, that's fine
 if ( $time_to_target <= $min_sleep )
 {
 // If required, sleep for a bit anyway
 sleep( $min_sleep );
 }
 else
 {
 // Sleep for the number of seconds until the target time
 sleep( $time_to_target );
 }
 }
?>

if you for some reason need a constant-time implementation of realpath(), try
<?php
function realpath_constant_time(string $path, float $target_seconds, bool &$constant_time_success = null){
 $start_time=microtime(true);
 $ret=realpath($path);
 $constant_time_success = @time_sleep_until($start_time+$target_seconds);
 return $ret;
}
?>

for example, a realtime that always uses exactly 1 millisecond (should be more than enough for SSD-based servers, perhaps rotating harddrive based servers may need something closer to 10 milliseconds, i don't know):
<?php
realpath_constant_time("/path/to/../to/file.txt",0.001,$constant_time_success);
?>

and you can use $constant_time_success to see if you needed more time (and thus failed to do realpath() in constant-time), or if you succeeded.