PHP connection mysql database problem

Database connection code:

<code>$server="localhost";
$username="root";
$password="";
$database="library";

$conn=mysqli_connect($server,$username,$password);
if (!$conn) {
    die("mysqli_connect failed".mysql_error());
}
$db_selected = mysql_select_db($database,$conn);
if (!$db_selected) {
    die("select db error:".mysql_error());
}</code>

The connection returns a prompt: select db error:
Why is this? Navicat can be used to connect to the library database.

Reply content:

Database connection code:

<code>$server="localhost";
$username="root";
$password="";
$database="library";

$conn=mysqli_connect($server,$username,$password);
if (!$conn) {
    die("mysqli_connect failed".mysql_error());
}
$db_selected = mysql_select_db($database,$conn);
if (!$db_selected) {
    die("select db error:".mysql_error());
}</code>

The connection returns a prompt: select db error:
Why is this? Navicat can be used to connect to the library database.

$db_selected = mysql_select_db($database,$conn);

Change to $db_selected = mysqli_select_db($database,$conn);

Yes, you wrote mysqli as mysql