A PHP Error was encountered Severity: Notice Message: Only variable references should be returned by reference Filename: core/Common.php

Question

I'm getting the following error in my Common.php file and hoping someone can give me a specific solution.

A PHP Error was encountered Severity: Notice Message: Only variable references should be returned by reference Filename: core/Common.php

Here is the code from Common.php

require_once 'Zend/Search/Lucene/Analysis/Analyzer.php';



{
    private $_filters = array();

    public function addFilter(Zend_Search_Lucene_Analysis_TokenFilter $filter)
    {
        $this->_filters[] = $filter;
    }

    /**
     * Apply filters to the token. Can return null when the token was removed.
     *
     * @param Zend_Search_Lucene_Analysis_Token $token
     * @return Zend_Search_Lucene_Analysis_Token
     */
    public function normalize(Zend_Search_Lucene_Analysis_Token $token)
    {
        foreach ($this->_filters as $filter) {
            $token = $filter->normalize($token);

            // resulting token can be null if the filter removes it
            if ($token === null) {
                return null;
            }
        }

        return $token;
    }
}