title: Interpolation (insert string directly) VS concatenation in the performance aspect
body: <p>Which is below method is faster when combining 2 strings ? And why can it run faster?</p>
  
  <p>PHP code:</p>
  
  <pre><code>$str1 = 'Hello';
  $str2 = 'World';
  </code></pre>
  
  <p>method 1:</p>
  
  <pre><code>$txt = $str1.$str2;
  </code></pre>
  
  <p>method 2:</p>
  
  <pre><code>$txt = "$str1$str2";
  </code></pre>