This appears to be two separate issues.
1. You are not getting new ticket alerts.
Please go to Admin Panel -> Settings. Scroll down to "Alerts and Notices".
Ensure that New Ticket is enabled. Ensure that at least one of the check boxes are checked under that. Which one? I personally use "Department Manager" however our department manager email is actually a email list that delivers mail to everyone in our IT department.
Additionally have you setup cron to process mail and/or are you piping mail to OST?
http://osticket.com/wiki/POP3/IMAP_Settings(http://osticket.com/wiki/POP3/IMAP_Settings)
2. You are seeing a warning.
This is a warning not an error. It is because PHP has changed something since OST was written, and the way the code is handling something is depreciate. This does not prevent it from working, but may (WILL) in the future when the backward compatibility is removed from PHP. This can be ignored for now, hopefully a new release of OST will fix this in the future.