title: How did the values 97 & 123 come?
body: <p>I am a beginner learning PHP. I was going through a post in Stackoverflow where an alphabet is shifted by 4 characters and the result is displayed. </p>
  
  <p>This is the code :</p>
  
  <pre><code>&lt;?php
  
      $code = 'string';
  
      $str = $code;
      $length = strlen($str);
      $result = '';
      $n = 0;
      for($i = 0; $i &lt; $length; ++$i){
          $c = ord($str[$i]);
          if (97 &lt;= $c &amp;&amp; $c &lt; 123) {                 
              $result.= chr(($c + $n + 7) % 26 + 97); 
          } else if(65 &lt;= $c &amp;&amp; $c &lt; 91) {            
              $result.= chr(($c + $n + 13) % 26 + 65);
          } else {
              $result.= $str[$i];
          }
      }
      echo $result;
  
  ?&gt;
  </code></pre>
  
  <p>However, I didn't understand the logic completely. How did values 97 and 123 come? 
  Assistance appreciated. </p>
  
  <p>Thanks a lot </p>