SWIG-PHP error: Invalid library

Question

Hello, I'm new to SWIG and I'm trying to make a PHP5 wrapper around this library https://sourceforge.net/projects/zinnia/

The project includes the interface file zinnia.i

Following www.swig.org/Doc1.3/Php.html

I run

swig -php -c++ zinnia.i
gcc `php-config --includes` -fpic -c zinnia_wrap.cpp
gcc -shared -L/usr/local/lib/ -lzinnia -o zinnia.so

Then I load it in PHP

extension=zinnia.so

But when I start apache I get this error

PHP Warning: PHP Startup: Invalid library (maybe not a PHP library) 'zinnia.so' in Unknown on line 0

Does anyone know how to solve it?

Apache/2.2.14 (Ubuntu) PHP/5.3.2 Swig 1.3.2

Thanks