title: Why strlen doesn't work in a paticular digit
body: <p>I made this function. It seemed it's working but when it comes to 20 digits number, the return value was 19. I'm wondering why this problem happen..</p>
  <p>My function</p>
  <pre><code>function sumDigits($n) {
      return strlen($n);
  }
  
  echo sumDigits(100);  //3
  echo sumDigits(1000);  //4
  echo sumDigits(12345);  //5
  echo sumDigits(1000000000);  //10
  echo sumDigits(145874589632);  //12
  echo sumDigits(0);  //1
  echo sumDigits(12345698745254856320);  //19  &lt;-- Why not 20?
  </code></pre>
  <p>Can you please somebody explain for me?</p>
  <p>Thank you so much.</p>