PHP 方法链接

谁能解释为什么这段代码不起作用（内容$this->_string）是空的？

<?php
class WordProcessor
{
    public $_string = '';
    public function __constructor($text)
    {
        $this->_string = $text;
    }
    public function toLowerCase()
    {
        $this->_string = strtolower($this->_string);
        return $this;
    }
    public function trimString()
    {
                echo $this->_string;
        $this->_string = trim($this->_string);
        return $this;
    }
    public function capitalizeFirstLetter()
    {
        $this->_string = trim($this->_string);
        return $this;
    }
    public function printResult()
    {
        echo $this->_string;
    }
}
$data = new WordProcessor("here Are some words!  ");
$data->trimString()->toLowerCase()->capitalizeFirstLetter()->printResult();