Im thinking about databases to use as a development environment for a cross platform php application.
What im ideally looking for is a database that supports the most correct standards of SQL, something i can write for and hopefully guarantee cross database compatibility when it comes to deployment.
I'd prefer it if it was open source.
Thanks
There really isn't such a thing - there is no "cross-platform SQL".
The SQL standards and specifications are very outdated and haven't kept in-step with advances with RDBMS systems, but also because different engines implement their own features (such as geospatial indexing and resultset paging) differently.
The best way to write a cross-platform application is to abstract away database-specific operations to a "data access layer" and implement this interface for each database backend you plan to support.
If you're doing very trivial operations such as SELECT (with simple joins), INSERT and UPDATE then you might get away with using a single set of SQL commands, assuming you don't run into issues with syntax or data-types (for example, JET Red/Access requires all dates to be delimited with # but T-SQL (SQL Server, Sybase) require single-quotes; or how MySQL uses backticks to escape names but T-SQL and JET Red uses [] square brackets. Also, each implementation has its own set of keywords).