title: How does the core 'php.ini' directive 'memory_limit' is considered as an 'Environment Variable' in an code example from the PHP Manual?
body: <p>I'm using <strong>PHP 7.2.8</strong> on my machine that runs on <strong>Windows 10 64-bit Operating System</strong>.</p>
  
  <p>I come across the following code example from <a href="https://secure.php.net/manual/en/configuration.file.php#configuration.file" rel="nofollow noreferrer">PHP Manual</a> having some description :</p>
  
  <blockquote>
    <p>Using environment variables can be used in <strong>php.ini</strong> as shown below.</p>
    
    <p><strong>Example #1 php.ini Environment Variables</strong></p>
  
  <pre><code>; PHP_MEMORY_LIMIT is taken from environment
  memory_limit = ${PHP_MEMORY_LIMIT}
  </code></pre>
  </blockquote>
  
  <p>I executed the above code on my machine it gave me the following error messages :</p>
  
  <pre><code>Warning: Use of undefined constant PHP_MEMORY_LIMIT - assumed 'PHP_MEMORY_LIMIT' (this will throw an Error in a future version of PHP) in demo.php on line 3
  
  Notice: Undefined variable: PHP_MEMORY_LIMIT in demo.php on line 3
  </code></pre>
  
  <p>I checked the list of available <strong>'Environment Variables'</strong> on my machine but I didn't get such environment variable named <code>$_ENV['PHP_MEMORY_LIMIT']</code></p>
  
  <p>Then, I checked the <strong>php.ini</strong> file present on my machine. I found the core <strong>php_directive</strong> <code>memory_limit=128M</code></p>
  
  <p>So, my question is why the code example from PHP Manual is considering something similar(<code>${PHP_MEMORY_LIMIT}</code>) to the core <strong>'php.ini'</strong> directive as an <strong>'Environment Variable'</strong>? </p>
  
  <p>Also, the code written in this code example doesn't work as such environment variable named <code>${PHP_MEMORY_LIMIT}</code> doesn't exist.</p>
  
  <p>I even checked <a href="http://git.php.net/?p=php-src.git;a=blob;f=php.ini-production;hb=HEAD" rel="nofollow noreferrer">the latest</a> <strong>php.ini</strong> from Git as recommended in the PHP Manual but in that file also I found the core <strong>php_directive</strong> <code>memory_limit=128M</code> and no environment similar to it.</p>
  
  <p>Is the PHP Manual having wrong code example?</p>
  
  <p>Someone please clear my doubts.</p>
  
  <p>Thanks.</p>