title: PHP 7.1 - Why no warning on void return value?
body: <p>The <a href="https://secure.php.net/manual/en/migration71.new-features.php#migration71.new-features.void-functions" rel="noreferrer">PHP manual notes</a> this in its overview of the new <strong>void</strong> return type added in PHP 7.1:</p>
  
  <blockquote>
    <p>Attempting to use a void function's return value simply evaluates to NULL, with no warnings emitted. The reason for this is because warnings would implicate the use of generic higher order functions.</p>
  </blockquote>
  
  <p>What does it mean by "<strong>warnings would implicate the use of higher order functions</strong>"?</p>