title: How to replace all commas without a space, but not commas with a space?
body: <p>Like, I could have done this if the second part of this question's title didn't exist:</p>
  
  <pre><code>$string = 'Hello,this is my string.';
  $replacedstring = str_replace(',', ', ', $string);
  </code></pre>
  
  <p>But what if the user did correctly format the sentence, as in <code>Hello, this is my string.</code>. Then that would result in it becoming <code>Hello,  this is my string.</code>, which I do not want.</p>
  
  <p>So, how would I do that using <code>preg_replace</code> and similar regex functions in PHP?</p>