How do I properly insert data into both tables, with one of them having a foreign key to the other table?

Currently, I've got these 2 tables in my database:

Users

id - Primary Key
fname
lname
email
password
level

Authorizations

id_network
id_user - FK to id on table Users
network

The PHP code that I was thinking of doing was something like this. And I am aware that this code is vulnerable to SQL injections, I will fix it, but I haven't done so yet.

$query = "INSERT INTO users (id, pnome, unome, email, pass, level) VALUES (DEFAULT, '$pnome', '$unome', '$email', DEFAULT, DEFAULT)";
$result = pg_query($dbconn, $query);

$query = "INSERT INTO authorizations (id_network, id_user, network) VALUES ('$id', ?????, 'Facebook')";
$result = pg_query($dbconn, $query);

My question is, what do I put in place of the question marks of the Authorization table insertion? Since there is the id_user foreign key to the Users table, and I wish to use the id from the entry that I just inserted into Users, how do I make sure that I am using that specific entry?