我的php crud（面向对象和pdo）更新不起作用。它显示了
致命错误：未捕获的pdoexception:sqlstate[hy093]：无效的参数号：未定义参数。在$stmtexec=$stmt->execute（）上；
我试图改变变量和参数，参数和间距，但仍然得到一个无效的结果。我怎样才能解决这个问题？

public function updateId($fields, $stud_id) {

    $st = "";
    $counter = 1;
    $total_fields = count($fields);

    foreach($fields as $key => $value){

        if($counter == $total_fields){

            $set = "$key = :".$key;
            $st = $st.$set;

        } else {

            $set = "$key = :".$key.", ";
            $st = $st.$set;
            $counter++;
        }
    }
    $sql = "";
    $sql.= "UPDATE students SET ".$st;
    $sql.= "WHERE id = ".$stud_id;
    $stmt = $this->connect()->prepare($sql);

    foreach($fields as $key => $value){
        $stmt->bindValue(':'.$key, $value);
    }
    $stmtExec = $stmt->execute();

    //if successful
    if($stmtExec){
        header('Location: index.hp');
    }
}