can i add PDO connection on mysql connection (dual)

I have question about PHP connection.

If you usually using MySQL connection, it'll easy to use. but my question is like this, what happen if i mix PDO connection with usual connection.

it was like :

mysql_connect('localhost','user','password');

mysql_select_db('database');

?>

$host = "localhost";
$user = "user";
$password = "password";
$database_name = "database";
$pdo = new PDO("mysql:host=$host;dbname=$database_name", $user, $password);
?>
 
in one file named connect.php
will it crash each other??
 
thank for your answer..