$host = "localhost";
$username ="user";
$password = "password";
$db = "db";
$charset="utf8";
//old
mysql_connect($host,$username,$password) or die("failed to connect");
mysql_select_db($db) or die("db not exists");
mysql_query("SET NAMES '".$charset."'") or die("failed to set collation");

//new
try{
$dsn = sprintf("mysql:host=%s;dbname=%s;charset=%s",$host,$db,$charset);
$pdo = new PDO($dsn,$username,$password);
$pdo->setAttribute(PDO::ATTR_ERRMODE,PDO::ERRMODE_EXCEPTION);
}catch(PDOException $e){
die($e->getMessage());
}


//old
$sql = "SELECT * FROM table WHERE foo = '".mysql_real_escape_string($_GET['foo'])."'";
$result = mysql_query($sql) or die("failed to execute sql ".mysql_error());
while($row = mysql_fetch_assoc($result)){
echo $row['field1'];
}

//new
$sql = "SELECT * FROM table WHERE foo = ".$pdo->quote($_GET['foo']);
try{
$statement = $pdo->query($sql);
}catch(PDOException $e){
die("failed to execute sql ".$e->getMessage());
}

while($row = $statement->fetch(PDO::FETCH_ASSOC)){
echo $row['field1'];
}