What is the purpose of returning $this in PHP
<pre><code>function select($select = '*')
    {
        if( is_string($select) ) {
            $select = explode(',', $select) ;
        }

        foreach($select as $s) {
            $s = trim($s) ;

            if($s != '') {
                $this-&gt;aSelect[] = $s ;
            }
        }

        return $this ;
    }
</code></pre>

<p>In PHP, what is meant by <code>return $this</code>.</p>
