Just curious "why" you would want to do this ...bit like reinventing the wheel......rather than just expose the existing standard apps through your intranet......even still, there are great third party solutions (Neptune or Worklogix comes to mind) that are fantastic web frontends for ESS that are easy to get in,up and going as replacements for the standard apps.
If you do choose the all custom route though, you are likely going to want to look using web services(wrapping many of the standard BAPIs for ESS) or possibly even using Gateway for OData/JSON calls and doing a heckuva lot of coding for the handling the exchange.
And to top it all off, you are developing in PHP which is according to many sources, on its way out as a top web dev language so you may be developing something considered "old" when all is said and done and will have to port it all again (part of the deal when you have custom apps....ongoing maintenance and support).
tldr;
Yes, it is possible....but not easy.
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