将您的事务代码包装在try-catch语句中.

Wrap your transaction code inside a try-catch statement.

//try {
$tags_input = array(6,4,5);
$conn = new PDO('mysql:host='.DB_HOST.';dbname='.DB_NAME.';charset=utf8',  
DB_USER, DB_PASSW, array(  
    PDO::ATTR_EMULATE_PREPARES => false,  
    PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,PDO::MYSQL_ATTR_INIT_COMMAND => "SET NAMES 'utf8'"));
} catch (Exception $e) {
  die("Unable to connect: " . $e->getMessage());
}    
try {  
    $conn->beginTransaction();   
    $sql = "INSERT INTO projects (id, pr_id, enabled) VALUES ( :val0, :val1, :val2)";
    $stmt = $conn->prepare($sql);  
    if(count($tags_input)>0){
            for($i = 0;$i<count($tags_input);$i++){
                    $stmt->bindValue(':val0', 57); 
                    $stmt->bindValue(':val1', $tags_input[$i]); 
                    $stmt->bindValue(':val2', 'Y'); 
                    $result = $stmt->execute();
            }
    }
$res1 = $conn->commit();    
} catch (Exception $e) {
  $conn->rollBack();
  echo "Failed: " . $e->getMessage();
}

编辑

Richard 提供了一个非常基础且直接的答案说明.

A really well-based and straight-forward explanation of the answer was provided by Richard as a comment.

出现错误的原因是因为您试图在已经关闭的事务上关闭该事务. beginTransaction 打开一个，然后 rollBack 或 commit 关闭它.您必须避免对单个 beginTransaction 语句执行两个操作，即提交/回滚，否则会出错.上面的try/catch代码可确保仅执行一个结束语句.

The reason you got error is because you were trying to close a transaction when it was already closed. beginTransaction opens one, and EITHER rollBack OR commit closes it. You have to avoid doing BOTH actions, meaning commit/rollback, for a single beginTransaction statement, or you'll get an error. The above try/catch code ensures that only one closing statement is executed.