下面的代码可以有效防止 sql 注入吗 ?

<code><?php

$dbh = new PDO("mysql:host=localhost; dbname=mydb", "root", "pass");

$dbh->setAttribute(PDO::ATTR_EMULATE_PREPARES, false); //禁用prepared statements的仿真效果
$dbh->exec("set names 'utf8'"); 

$sql="select * from table where username = ? and password = ?";

$query = $dbh->prepare($sql); 

$exeres = $query->execute(array($username, $pass)); 

if ($exeres) { 

    while ($row = $query->fetch(PDO::FETCH_ASSOC)) {
        print_r($row);
    }
    
}
$dbh = null;

?>
</code>

<code><?php

$dbh = new PDO("mysql:host=localhost; dbname=mydb", "root", "pass");

$dbh->setAttribute(PDO::ATTR_EMULATE_PREPARES, false); //禁用prepared statements的仿真效果
$dbh->exec("set names 'utf8'"); 

$sql="select * from table where username = ? and password = ?";

$query = $dbh->prepare($sql); 

$exeres = $query->execute(array($username, $pass)); 

if ($exeres) { 

    while ($row = $query->fetch(PDO::FETCH_ASSOC)) {
        print_r($row);
    }
    
}
$dbh = null;

?>
</code>

<code>......

$sql="select * from table where username = ?";

......


    while ($row = $query->fetch(PDO::FETCH_ASSOC) && $row['pass'] == $pass) {
        print_r($row);
    }
</code>