我可以在 PHP 中混合 MySQL API 吗？

我已经搜索了网络，到目前为止，我看到的是，你可以使用 mysql_和 mysqli_一起意味着:

<?php
$con=mysqli_connect("localhost", "root" ,"" ,"mysql");


if( mysqli_connect_errno( $con ) ) {
echo "failed to connect";
}else{
echo "connected";
}
mysql_close($con);
echo "Done";
?>

或者

<?php
$con=mysql_connect("localhost", "root" ,"" ,"mysql");
if( mysqli_connect_errno( $con ) ) {
echo "failed to connect";
}else{
echo "connected";
}
mysqli_close($con);
echo "Done";
?>

是有效的，但是当我使用这个代码时，我得到的是:

Connected
Warning: mysql_close() expects parameter 1 to be resource, object given in D:\************.php on line 9
Done

对于第一个和相同的除了与 mysqli_close()。对于第二个。

What is the problem? Can't I use mysql_ and mysqli together? Or is it normal? Is the way I can check if the connections are valid at all? (the if(mysq...))