title: Replace single line break with double line break
body: <p>I have a string:</p>
  
  <pre><code>$str =
  'hello
  world';
  </code></pre>
  
  <p>And I want to replace single line breaks to double line breaks, so we have:</p>
  
  <pre><code>$str =
  'hello
  
  world';
  </code></pre>
  
  <p>I tried: </p>
  
  <pre><code>$str = str_replace("\n", "\n\n", $str);
  </code></pre>
  
  <p>But it did not work. What's the correct approach? Can we use <code>PHP_EOL</code>...?</p>