Writing cron to sync users tables

[kpmedia]

I need help writing a cron job (PHP file in vBulletin to query the datbase).

Forum #1
- 001, admin, admin@site.com, ...
- 002, john, john@site.com,
- 007, sally, sally@sally.com.
- 010, me, me@me.me
- 014, steven, steve@steve.com
- 015, jimmy, james@me.com

Forum #2
- 001, admin, admin@site.com
- 003, john, john@site.com
- 005, sally, sally@sally.com
- 010, me, me@notme.com

Users may have different emails, different user IDs, and be on the first forum only.

What I have is two sites, two databases, and I want to "share" the users -- or rather, sync them!
I can run the cron every 5 minutes if needed.
Note: This is a dedicated server.
Both sites are already established; these are not new.
Site #1 has about 20,000 members. Site #2 has under 5,000.
For this to work, I can disabled new registrations on #2 by having the register.php 301 redirect to #1's page.
To not tank the server, I'm thinking the cron will need to be batch-limited (100?) in some way (add field with counter?)

User IDs will be ignored. This would totally hose both forums if it touched userID!!!
The username is the primary field to run the cron against.

It needs to do 3 things
(1) Check for email changes,
(2) Add new users from #1 to #2, including passwords, email, username (don't think I care about anything else)
(3) Sync password changes

I don't think I left anything out.

Any help you can think of would be great, even if it's partial. I can do some myself, but need input.

As always, thanks for reading. And hopefully, if you can assist, I can thank you for helping.

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