If the value of username is:

 $_POST['user'] = "1' OR 1 LIMIT 1; --";

Then the mysql query becomes:

select * 
from zend_adminlist 
where user_name = '1' OR 1
 LIMIT 1; 
 --' and password = '$pass'

If the value of username is:

 $_POST['user'] = "1' OR 1 LIMIT 1; --";

Then the mysql query becomes:

select * 
from zend_adminlist 
where user_name = '1' OR 1 LIMIT 1; --' and password = '$pass'

If the value of username is:

 $_POST['user'] = "1' OR 1 LIMIT 1; --";

Than the query become:

select * 
from zend_adminlist 
where user_name = '1' OR 1
LIMIT 1; 
--' and password = '$pass'

If the value of username is:

 $_POST['user'] = "1' OR 1 LIMIT 1; --";

Then the mysql query becomes:

select * 
from zend_adminlist 
where user_name = '1' OR 1
LIMIT 1; 
--' and password = '$pass'