Description:
------------
When mysqli_poll is given a single link in its read array, and the async query on that link runs longer than the timeout, the timeout expires and poll returns (correctly) that nothing is ready to read.

However, in the same scenario when there are multiple links to read, mysqli_poll ignores the timeout and returns only when the longest running query resolves.

The man page is light on this function, so maybe I am misunderstanding how timeout should work.  Also, there's further discussion at this SO post: http://stackoverflow.com/a/32576278/2908724

Test script:
---------------
$link0  = mysqli_connect(...);
$link1  = mysqli_connect(...);
$link2  = mysqli_connect(...);

$sql0   = 'SELECT SLEEP(2) AS wait, 1 AS num';
$sql1   = 'SELECT foo FROM';
$sql2   = 'SELECT 2 AS num';

mysqli_query($link0, $sql0, MYSQLI_ASYNC);
mysqli_query($link1, $sql1, MYSQLI_ASYNC);
mysqli_query($link2, $sql2, MYSQLI_ASYNC);

$links  = array ($link0, $link1, $link2);
$begin = microtime(true);
$i = 0;
do {
    printf("start i=%d @ T+%.f\n", $i, (microtime(true)-$begin));
    $read = $error = $reject = $links;
    $count = mysqli_poll($read, $error, $reject, 1, 500000);
    if (0 < $count) {
        foreach ($links as $j => $link) {
            $result = mysqli_reap_async_query($link);
            if (is_object($result)) {
                printf("link #%d, row=%s\n", $j, json_encode($result->fetch_assoc()));
                mysqli_free_result($result);
            } else if (false !== $result) {
                printf("link #%d, output=%s\n", $j, $link);
            } else {
                printf("link #%d, error=%s\n", $j, mysqli_error($link));
            }
        }
    }
    printf(
        "finish i=%d, count(read, error, reject)=(%d, %d, %d) @ T+%f\n\n",
        $i++, count($read), count($error), count($reject), (microtime(true)-$begin)
    );
} while (count($links) !== count($read) + count($error) + count($reject));


Expected result:
----------------
start i=0 @ T+0.000000
link #0, row=null
link #1, error=You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '' at line 1
link #2, row={"num":"2"}
finish i=0, count(read, error, reject)=(1, 1, 0) @ T+1.500000

start i=1 @ T+1.500000
link #0, row={"wait":"0",a"num":"2"}
link #1, row=null
link #2, row=null
finish i=0, count(read, error, reject)=(1, 0, 2) @ T+2.000000


Actual result:
--------------
start i=0 @ T+0.000002
link #0, row={"wait":"0","num":"1"}
link #1, error=You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '' at line 1
link #2, row={"num":"2"}
finish i=0, count(read, error, reject)=(1, 0, 0) @ T+2.001756

start i=1 @ T+2.001827
finish i=1, count(read, error, reject)=(0, 0, 3) @ T+3.503024