compiling php5-interbase extension from source
I assume that php5 is already installed and we have the source for php5.2.x
$cd php-5.2.6/ext/interbase
$phpize or /opt/php5.2.x/bin/phpize (if is installed there)
$./configure --with-php-config=/opt/php5.2.x/bin/php-config
$make ; sudo make install
add this extension to php.ini (at the end for example)

extension=interbase.so

restart apache
$sudo /etc/init.d/apache2 restart

Load phpinfo() and search for interbase extension