Let’s talk the standard way - It would be a great option to opt for creating a REST based ODATA services. You can expose the SAP functionalities mainly BAPI or any RFC as a service with SAP gateway. SAP gateway lets applications to exchange data across a variety of technologies and applications.
In case the requirements are not extensive and can be catered by existing API’s exposed by SAP, then you can just enable the required services for usage. These APIs will require good hands-on knowledge of SAP Services.
You can build an SAP source data plugin using JAVA and JAVA SAP Connector as it is widely accepted solution and works well within PHP environment.
There are third party API’s available (Process Runner Lite) as well but they come at a cost. You can estimate the cost of the effort and choose the optimal way.