You can communicated with any SAP system from PHP in many ways, but the preferred standard available choices are
PHP has got one RFC library to communicate with SAP. But the main job in your problem statement lies with your partner as they are the one dealing with SAP component. You need to check with them what they prefer services or RFC. Don’t forget to double check with them in case they already have any existing API (can be anything) which can serve your purpose. Because it entirely depends on them as in how they want you to access their data.
Also, the fetching of data can be done n times as per requirement, It entirely depends upon when the data is getting updated and refreshed. You can configure calls (either RFC or services) with some scheduled Jobs to run only once a day