From an architecture point of view, one of the current best practice approaches for connecting an SAP to a non-SAP system is called a Services Oriented Architecture. Although SOA can be defined through a number of different technical implementations, at a high level SOA means exposing reusable services through a generic middleware layer. This middleware can be written in any language that is compatible with your application subsystems.
Of course, my background is Java integration for SAP so I would recommend looking at SAP's Java connector, JCo. JCo allows you to tap into any exposed RFC/BAPI interface available within SAP. The major caveat when exposing SAP tables this way is to do so through a published and supported SAP interface. If you attempt direct table updates against a standard SAP table you are almost guaranteed failure. JCo is available through SAP service marketplace as a free download, and sites like SearchSAP.com have a number of helpful articles to get you going.
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