Basics of PHP Frameworks

I have never used a framework before but have played with PHP for a little over 10 years.
I have been told that Frameworks are really good and I should look at it and I want to take the step for a small project I am just about to start - BUT, I have 3-4 weeks and I’m not sure if I can learn how to use a framework and build the app within the required timeframe.
So…
How much time should I set aside to bring myself up to speed with CodeIgniter?
How is CodeIgniter better than any other framework (or is it much of a muchness)?
Below is a list of functions I need to build into the app - how much would already exist in the framework?

1. Check IMAP email Inbox
   
2. Get message and check for attachment
   
3. Extract attachment and read contents of text file
   
4. Re-format and export into CSV
   
5. SFTP CSV to another server
   
6. Send CSV as attachment
   

I am more asking to understand how much it will help with to understand it’s capabilities because I still don’t fully understand the benefits (most benefits spruiked in features don’t make a lot of sense to me)
THanks