PHP PDO无法在类中绑定参数

我无法获得将params绑定到Mysql查询的代码，我做错了什么？

错误

致命错误：未捕获的异常"PDOException"，消息为"SQLSTATE[HY093]"：无效参数编号：绑定变量的数量与/Users/warren/Sites/Frame-Work/Modules/User Login/classes/db.php:34堆栈跟踪：#0/Users/warren/Sites/Frame Work/Modules/User Login/classes/db.phpLogin/classes/db.php（48）：dbConnect->get_contents（）#2｛main｝在第34行的/Users/warren/Sites/Frame-Work/Modules/User Login/classes/db.php中抛出

代码

<?php 
include '../../../../config.php';
$dbUser = $config['username'];
$dbPass = $config['password'];
class dbConnect {
    public function connect($dbUser,$dbPass) {
        $this->dbUser = $dbUser;
        $this->dbPass = $dbPass;
        try {
            $this->connect = new PDO('mysql:hostname=localhost;dbname=totalrisk', $this->dbUser, $this->dbPass);
            $this->connect->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
            return $this->connect;
        } catch (Exception $e) {
            echo 'ERROR: ' . $e->getMessage();
        }
    }
    public function select_table($table) {
        $this->select_table = $table;
        echo $this->select_table;
    }
    public function select_col($col) {
        $this->select_col = $col;
        echo $this->select_col;
    }
    public function get_contents() {
        $stmt = $this->connect->prepare('SELECT * FROM :table');
        $stmt->bindParam(':table', $this->table , PDO::PARAM_STR);
             $stmt->execute();
             $result = $stmt->fetchAll();
             while($row = $stmt->fetch()) {
                print_r($row);
             }
    }
}
$conn = new dbConnect();
$conn->connect($dbUser,$dbPass);
$conn->select_table('users');
$conn->select_col('site_name');
$conn->get_contents();
?>