mysqli bind_param

Question

lets pretend we have a table 'item' with fields 'id' and 'num' and also a code like below that doesn't work.

$db = new mysqli('localhost', 'user', 'pass', 'db') ;
if (!$st  =  $db->prepare('select id from item')) die($db->error) ;
if (!$st2 =  $db->prepare('update item set num = 1 where id = ?')) die($db->error) ;

$st->execute() ;
$st->bind_result($id) ;

while ($st->fetch()) {

    $st2->bind_param('i', $id) ;
    $st2->execute() ;
    echo $id.'<br/>' ;
}

It just print out something like^ 1 2 3 but no changes takes plase id the database ($st2->affected_rows equals zero). What's wrong with it?

PS Not the real code, but it totally describes the problem.

Answer 1

What's wrong with it?

For some reason you do no error checking for execute.

$st2->execute() or trigger_error($db->error);

will tell you if there was an error with query.
if there was none - check the way you're checking your updated values.

by the way, better version of your code

$dsn = 'mysql:host=localhost;dbname=db;charset=utf8';
$opt = array(
    PDO::ATTR_ERRMODE            => PDO::ERRMODE_EXCEPTION,
    PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC
);
$pdo = new PDO($dsn,'user','pass', $opt);

$sth = $db->prepare('select id from item');
$sth->execute();
$ids = $sth->fetchAll();

$sth = $db->prepare('update item set num = 1 where id = ?');
foreach ($ids as $row) {
    $sth->execute($row['id']);
}

Answer 2

According to the documentation bind_result has to be called before execute :

$st->bind_result($id) ;
$st->execute() ;
while ($st->fetch()) {

    $st2->bind_param('i', $id) ;
    $st2->execute() ;
    echo $id.'<br/>' ;
}