Best way to access Exchange using PHP?
Update as of 2020:
Over a decade since this question and things have moved on. Microsft now has a Rest API that will allow you to easily access this data.
Original Answer
I have not used PHP to do this but have experience in using C# to achieve the same thing.
The Outlook API is a way of automating Outlook rather than connecting to Exchange directly. I have previously taken this approach in a C# application and it does work although can be buggy.
If you wish to connect directly to the Exchange server you will need to research extended MAPI.
In the past I used this wrapper MAPIEx: Extended MAPI Wrapper.
It is a C# project but I believe you can use some .NET code on a PHP5 Windows server. Alternatively it has a C++ core DLL that you may be a able to use. I have found it to be very good and there are some good example applications.
Sorry for the delay no current way to keep track of posts yet.
I do agree adding more layer on to your application and relying on 3rd party code can be scary (and rightfully so.)
Today I read another interesting post tagged up as MAPI that is on a different subject. The key thing here though is that it has linked to this important MS article. I have been unaware of the issues until now on using managed code to interface to MAPI although the C++ code in the component should be unaffected by this error as it is unmanaged.
This blog entry also suggests other ways to connect to MAPI/Exchange server. In this case due to these new facts http://us3.php.net/imap may be the answer as suggested by the other user.
Is your customer using Exchange 2007? If so, I'd have a look at Exchange Web Services. If not, as hairy as it can be, I think WebDAV is your best bet.
Personally I don't like using the Outlook.Application COM object route, as its security prompts ("An application is attempting to access your contacts. Allow this?", etc.) can cause problems on a server. I also think it would be difficult to accomplish your impersonation-like tasks using Outlook, such as sending mail as a given user.
I have released an open-source MIT licensed library that allows you to do some basic operations in PHP using Exchange Web Services.
I have only tested it on Linux but I don't see any reason why it wouldn't work on a Windows installation of PHP as well.