You can do this with pcntl_alarm() and pcntl_signal() to send yourself the SIGALRM POSIX signal after a 
number of seconds.

<?php

declare(ticks = 1);

function handle_timeout() {
  throw new Exception('Semaphore acquire timeout');
}

pcntl_signal(SIGALRM, 'handle_timeout');

$key = ftok(__FILE__, 'R');
$sem = sem_get($key);

pcntl_alarm(10);
sem_acquire($sem);
pcntl_alarm(0);

echo 'Semaphore acquired :)';
sem_release($sem);

As of PHP 5.6.1 the $nowait parameter is available, which appears
to provide an even more flexible solution to the timeout problem.

Do you agree?

@jhill9693: The suggested workaround with pcntl_alarm doesn't work; probably because of #49340 (https://bugs.php.net/bug.php?id=49340).

@cmb: $nowait would stop trying to acquire the semaphore immediately. In my case I want to wait for the semaphore a few seconds and fail if it couldn't be acquired. To repeatedly call sem_acquire with $nowait until the semaphore is either acquired or a specified time period is elapsed would be not a good solution.

Any chance of this being implemented?
There is semtimedop() system call that has a timeout parameter.