PDO sql语句和常规sql之间的区别？

I've learned PHP from a book and its told me to use PDO objects or sql statements (I'm not sure if that's the right terminology, I apologize if it's not).

When I look up sql stuff, a lot of the times I see stuff like this:

$link = mysql_connect('localhost', 'mysql_user', 'mysql_password');
if (!$link) {
    die('Could not connect: ' . mysql_error());
}
if (!mysql_select_db('database_name')) {
    die('Could not select database: ' . mysql_error());
}
$result = mysql_query('SELECT name FROM work.employee');

But in my code and in the book, I'm doing stuff like this:

global $db;
    $query = "SELECT * FROM users WHERE username='$username'";
    $results = $db->query($query);
    $results = $results->fetch();

What's the difference between these two 'styles'?

First the function those are mysql_* (like mysql_query, mysql_connect etc) are deprecated and will not be supported in PHP future versions. So PDO or Mysqli are preferred way of communication with database.

The PDO 's prepared statements are used for avoiding SQL injection attacks. like in normal mysql_query you will use like this

$query = "SELECT * FROM users WHERE username='$username'";
$results = mysql_query($query);

but in PDO you have to use like this

$params = array(':username' => 'test', ':email' => $mail);

$pdo->prepare('
   SELECT * FROM users
   WHERE username = :username
   AND email = :email');

$pdo->execute($params);

So PDO is recommended way. For more detail you can refer to

http://wiki.hashphp.org/PDO_Tutorial_for_MySQL_Developers

http://php.net/pdo

http://net.tutsplus.com/tutorials/php/why-you-should-be-using-phps-pdo-for-database-access/

The first style was written long ago, or was written by people who stopped learning PHP before PHP5 came out. mysql_query is deprecated, and has been for a while now, and you should never be using it in a new project.

The second is using PDO, one of the newer database APIs. PDO supports a bunch of things that make working with SQL easier.

It's still pretty hideous as written, though. Most people would recommend using parameterized queries (a form of prepared statements) to separate the data from the SQL. This helps prevent "SQL injection", a process by which someone feeds you data that tricks your database into executing queries you never intended for it to.