All three major DB2 platforms (z/OS, i, and LUW) share enough common SQL syntax to make cross-platform application development an approachable goal. It's worth pointing out that the current edition of IBM's SQL Reference for Cross-Platform
Development contains over a thousand pages.
IBM Data Studio, a no-cost IDE based on Eclipse, is capable of running SQL statements, browsing database objects, and developing stored procedures on every DB2 platform. It is a separate download from DB2 servers and clients.
Unlike cross-platform DML, which is largely identical, an application's underlying DDL could differ considerably between DB2 for i and DB2 LUW, the platform/family to which DB2 Express-C belongs.
Cross-platform DB2 connectivity is most easily accomplished via IBM's JDBC Type 4 driver, but it might not be practical for PHP. Accessing DB2 for i (and z/OS) over other protocols requires an extra piece of software called DB2 Connect.