title: PHP - strpos didn't check first word
body: <p>I'm trying to do some validation in PHP, and one of them is checking whether there is a specific word in the inputted string or not.
  The problem is, my code seem to not working when I put the specified word first.</p>
  
  <p>here's the code:</p>
  
  <pre><code>$word = "aa bb cc dd";
  if(strpos($word, 'aa') == false)
  {
      echo "wrong input";
  }
  </code></pre>
  
  <p>but if I change the <code>$word</code> to either <code>bb aa cc dd</code> or <code>bb cc dd aa</code>, it works. I wonder how to fix this though.</p>