Auto increment php variable from database for different users

I have this code:

mysql_query("UPDATE users SET admin='1'");

Code (markup):

But now instead of it just making admin='1' I want it to make admin equal to its current value+1 this will need to happen for all users.

eg.
User 1 that has admin=0 will become admin=1
User 2 that has admin=1 will become admin=2

How can I do achieve this?

 

UPDATE users SET admin=admin+1

 

Clarify :

mysql_query("UPDATE users SET admin='admin+1'");

PHP:

 

Thanks it worked, I added ' ' which made it failed but without it, it worked.

Thanks