title: APCU doesn't store in PHP 5.6
body: <p>I'm using APCU for data caching. It was working fine until I've updated server. After updating I have those versions:
  Apache - 2.4.10
  PHP - 5.6
  APCU - 4.0.11</p>
  
  <p>Now nothing is stored in cache. When I run php -r 'apcu_store("Test", "Test value");' it returns "1" but consequent apcu_fetch("Test") returns nothing. </p>
  
  <p>What could be broken?</p>
  
  <p>APCU is configured as following:</p>
  
  <pre><code>APCu Support =&gt; Enabled
  Version =&gt; 4.0.11
  APCu Debugging =&gt; Disabled
  MMAP Support =&gt; Enabled
  MMAP File Mask =&gt;
  Serialization Support =&gt; php
  Revision =&gt; $Revision: 328290 $
  Build Date =&gt; Dec  1 2016 12:26:37
  
  Directive =&gt; Local Value =&gt; Master Value
  apc.enabled =&gt; On =&gt; On
  apc.shm_segments =&gt; 1 =&gt; 1
  apc.shm_size =&gt; 2048M =&gt; 2048M
  apc.entries_hint =&gt; 4096 =&gt; 4096
  apc.gc_ttl =&gt; 3600 =&gt; 3600
  apc.ttl =&gt; 0 =&gt; 0
  apc.smart =&gt; 0 =&gt; 0
  apc.mmap_file_mask =&gt; no value =&gt; no value
  apc.enable_cli =&gt; On =&gt; On
  apc.slam_defense =&gt; On =&gt; On
  apc.rfc1867 =&gt; Off =&gt; Off
  apc.rfc1867_prefix =&gt; upload_ =&gt; upload_
  apc.rfc1867_name =&gt; APC_UPLOAD_PROGRESS =&gt; APC_UPLOAD_PROGRESS
  apc.rfc1867_freq =&gt; 0 =&gt; 0
  apc.rfc1867_ttl =&gt; 3600 =&gt; 3600
  apc.preload_path =&gt; no value =&gt; no value
  apc.coredump_unmap =&gt; Off =&gt; Off
  apc.use_request_time =&gt; On =&gt; On
  apc.serializer =&gt; php =&gt; php
  apc.writable =&gt; /tmp =&gt; /tmp
  </code></pre>