title: why stop word removal be null? (php)
body: <p>I'm the beginner NLP programmer in PHP.
  I just want to discuss about the stop word removal.</p>
  
  <p>this is my practice:</p>
  
  <p>I have the following declaration of a variable <code>$words = "he's the young man";</code></p>
  
  <p>and then I remove the common words like this</p>
  
  <pre><code> $common_words = $this-&gt;common_words();
   $ncwords = preg_replace('/\b('.implode('|',$common_words).')\b/','',$data); 
   // I have save the array common_words in another function
  </code></pre>
  
  <p>and I explode my no common words</p>
  
  <pre><code>$a_ncwords=explode(" ", $ncwords);
  </code></pre>
  
  <p>But, when I print <code>$a_ncwords</code>, like so <code>print_r($a_ncwords);</code></p>
  
  <p>I get a result like this:</p>
  
  <pre><code>Array ( [0] =&gt; [1] =&gt; [2] =&gt; young [3] =&gt; man )
  </code></pre>
  
  <p>why are the <code>index[0]</code> and <code>index[1]</code> array values null?</p>