I have wrote the steps in compiling php_interbase.dll with using the Firebird 2.5 binaries here but if you want to debug the Firebird client dll or the engine you should follow these steps (requires full Firebird 2.5.x compilation)

If you want to build only Firebird 2.5 with a free beer compiler i have created a video where i describe and show the steps that you need to follow

http://vimeo.com/14190827

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