usort回调函数无效名称

Question

我正在尝试根据他们的highCharCount对一个单词对象数组进行排序（找到具有highCharCount最高值的单词对象）。出于某种原因，我收到此错误消息：

Warning: usort() expects parameter 2 to be a valid callback, function 'compare' not found or invalid function name in C:\xampp\htdocs\practicePhp\classes.php on line 48

我正在阅读OO PHP usort issue，但我认为它应该对我有用，因为usort和compare函数属于同一个类。我不确定为什么它找不到“比较”。

class SentenceContentContainer {
    public $strSentence; //theSentence entered
    public $arrayOfWords = []; //words in the sentence
    private $num_words_in_sentence; 

    function __construct($strSentence)
    {
        $this->strSentence = $strSentence;
        $this->arrayOfWords = explode(" ", $strSentence); //get the array of words in the string
        $num_words_in_sentence = count($this->arrayOfWords); //count elements in the sentence
    }

    //function addWordToContainer(&$wordToAdd)
    //{
    //  if($arrayOfWords == null)
    //      $arrayOfWords = new array[]
    //  array_push($arrayOfWords, $wordToAdd);
    //}

    function sortHighestLetterRepeaterOfSentence()
    {
        if($this->arrayOfWords != null) {
            usort($this->arrayOfWords, "compare");

            //print it out
            foreach ($this->arrayOfWords as $key =>$value) {
                echo"$key: $value\n";
            }
        }
    }

    function compare($word1, $word2)
    {
        if ($word1->highCharCount == $word2->highCharCount) {
            return 0;
        }
        return ($word1->highCharCount < $word2->highCharCount) ? -1 : 1;
    }
} //SentenceContentContainer