Pass variable between PHP files

Question

That may be a duplicate question, but I have this problem. I took a project to continue the service here and it uses old functions like mysql_connect, mysql_querry and I need to change it. ex:

mysql_connect("localhost", "root", "",) or die ("Falha ao conectar ao banco de dados - ".mysql_error());

And it used a file with a function to make a connection to the database, and mysql_query did not need to pass a connection as parameter so it worked, since mysqli_query needs a connection, then every time I am using I have to open a new connection and then close at the end of mysqli_query.

Finally I wanted to know how to keep this connection file and send this variable $con to all the files you need. example:

$con = mysqli_connect("localhost", "root", "","db") or die ("Falha ao conectar ao banco de dados - ".mysqli_error($con));