Description:
------------
apache-2.2.31 and PHP 7.0.3 and 5.6.18, pecl-apcu 5.1.3 on Gentoo Linux 4.1.15 x86-64 tested

A thumbnail script is called 36 times from a gallery web page generating and storing a thumbnail using imagick/gd and pecl-apcu and serving it either as webp or jpeg depending on HTTP_ACCEPT.
Sometimes (~1 in 200 thumbnails) the script will serve webp to a non-webp browser if it has been loaded/cached in a webp browser before (e.g. go to gallery with Opera/WebKit and then using Firefox).
After investigating (echoing back the HTTP_ACCEPT) it appears that the script receives the HTTP_ACCEPT from an older Opera/WebKit (webp) request, even though Firefox console shows it never sent that header.

Firefox console shows it sent "Accept: image/png,image/*;q=0.8,*/*;q=0.5" and PHP echos back "X-Accept: image/webp,image/*,*/*;q=0.8" which was sent by Opera/WebKit seconds before that.

Maybe OpCache/APCu related? Or some threading/sharing problem?

Test script:
---------------
<?php

$format = isset($_SERVER['HTTP_ACCEPT']) && stripos($_SERVER['HTTP_ACCEPT'], 'image/webp') !== false ? 'webp' : 'jpeg';

// sometimes this is data from an old request of a different browser!!!
header('X-Accept: ' . $_SERVER['HTTP_ACCEPT']);

// create, output and store (APCu) thumb in $format

?>

Expected result:
----------------
$_SERVER['HTTP_ACCEPT'] should contain what the browser actually sent and thus

Actual result:
--------------
$_SERVER['HTTP_ACCEPT'] sometimes contains a foreign request made by a different browser sometime ago