Generating new ID: Getting actual IDs - php

Question

I'm working on a login module in php and facing some problem generating ids.
On the registration part, I need to generate an unique ID per user. I thought I'd be able to generate this ID by incrementing the last ID in the database by 1, but it doesn't seem to work.
Here's my code:
$user = $_POST['post_username'];
$pass = $_POST['post_password'];
function generateID(){
$aux = mysql_query("SELECT usr_id FROM users");
$i = 0;
$resulArray = mysql_fetch_row($aux) or die("Error fetching!");
$n = mysql_num_rows($aux);
$b = $resulArray[$i];
$c = $b;
for($i = 0; $i < $n; $i = $i +1){
$b = $resulArray[$i];
if($b >= $c) $c = $b;
}
return $c + 1;}
Here's my MySQL structure:
username -> String
password -> String
usr_id -> int

Answer 1

Don't do this. It's unreliable and cannot address race conditions. Just use an auto-increment primary key:
CREATE TABLE users
id unsigned int auto_increment primary key,
...
);
then
INSERT INTO users (id, ....) VALUES (null, ...)
and
$userID = mysql_last_insert_id();
This is inherently reliable, handles race conditions, and will guarantee unique ids for this table. You won't need any custom verification, just auto-increment the id field and let MySQL take care of the ids.