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The SQL standards and specifications are very outdated and haven't kept in-step with advances with RDBMS systems, but also because different engines implement their own features (such as geospatial indexing and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).
MySql Why ?
MySQL :: The world's most popular open source database (RDBMS)[6] as of 2008 that runs as a server providing multi-user access to a number of databases.
Features
As of April 2009, MySQL offered MySQL 5.1 in two different variants: the open source MySQL Community Server and the commercial Enterprise Server. MySQL 5.5 is offered under the same licences.[32] They have a common code base and include the following features: A broad subset of ANSI SQL 99, as well as extensions
& so many ... Source
or little one & faster postgres Also Why ?
because it is PostgreSQL: The world's most advanced open source database