title: phpdbg/phpunit throws error even when memory limit is -1
body: <p>I have a 32GB machine and running over 2000 test cases using phpdbg for code coverage.</p>
  
  <pre><code>phpdbg -qrr -d memory_limit=-1./vendor/phpunit/phpunit/phpunit --debug --verbose
  </code></pre>
  
  <p>After running for some time, it throws the following error even <code>memory_limit</code> is -1</p>
  
  <blockquote>
    <p>map() failed: [12] Cannot allocate memory
    [ PHP Fatal error:  Out of memory (allocated 5830606848) (tried to allocate 65536 bytes) ]</p>
  </blockquote>