如何理解更新语句是否成功运行？

In my program I want update some information, so I used:

 return ($result->rowCount() == 1)? true: false;

this way, if you save information without any change, false is returned and this is not the result usually we expect.

I change my function to this one

try{
        $result=$db->prepare($sql);
            $result->execute($arr);
            return true; 
        }catch(Exception $e){
            return false;   
        } 

Is it the best way? Does this way guarantee that the update statement worked or not?

Assuming your $db is instance of PDO

When you run PDO::prepare you can detect sql syntax error and when you run PDO::execute you will be sure that syntax is correct. Then application is resposible for knowing if update should have updated something or not.

So here is some sample code:

function update($db, $sql)
{
    $preparedSql = $db->prepare($sql);

    if(!$preparedSql) {
        // syntax error occured
        $errorInfo = $db->errorInfo();
        // handle error
        return false;
    }

    $db->execute($preparedSql);
    // note that rowCount returns 0 when none updated
    return $db->rowCount();
}

In your application

$result = update($db, $sql);

if($result === false)
{
     // error occured
} elseif($result === 0) {
     // zero rows updated
} else {
     // some rows were updated
}

Note that it's not tested and for learning purposes only, update function should accept to bind parameters for real world application