Thanks for contacting support.
Our .NET APIs are based on .NET framework and can be used in any application build on top of .NET Framework (Desktop, Web etc). Whereas our Java APIs are developed on top of JRE and it can be used in any application (desktop or web) based on Java platform. Whereas our Cloud APIs are REST and provide document generation, conversion and automation platform for the developers in cloud. It makes easier for the Web and Mobile application developers to work with Microsoft Word documents, Excel spreadsheets, PowerPoint presentations, Adobe PDFs, OpenDocument formats, Barcodes, OCR and Email formats & protocols in their apps. The REST API can be called from any platform: .NET, Java, Ruby, Salesforce, and Amazon etc.
You may use COM Interop to use .NET APIs in PHP or you may consider using Cloud APIs as they support PHP platform.
Please ask the query in GroupDocs support forum and respective team will be able to answer it accordingly.