HowTo: Install Firebird/Interbase with PHP on Windows. A step-by-step tutorial.

• Copy C:\firebird\bin\fbclient.dll to your C:\windows\system32 directory and rename the file to gds32.dll
• Copy php_interbase.dll from your php\ext directory to the C:\windows\system32 directory.
• Copy libeay32.dll and ssleay32.dll from your php directory to the C:\windows\system32directory.
• Edit your php.ini, un-comment the line
  extension=php_interbase.dll
  (you have to remove the semicolon from that line).
• Restart your webserver (Apache).