PHP Startup: Invalid library (maybe not a PHP library) 'oci8.so' in Unknown on line 0

Hello, I need to access an oracle database from php. The server is
running php 5.2.5, then I downloaded the source package used to
compile this version.

I downloaded also oracle-instantclient11.1-basic-11.1.0.7.0-1.i386.rpm
and oracle-instantclient11.1-devel-11.1.0.7.0-1.i386.rpm

After some time tewaking with ./configure of php, I managed to compile
with ./configure --with-oci8=shared,instantclient

I want to compile oci8 as shared library so I can continue using the
same binaries and only add a .so to the extensions directory, but I
has a problem now. When starting, php reports this error:

PHP Warning:  PHP Startup: Invalid library (maybe not a PHP library)
'oci8.so'  in Unknown on line 0

Checking the mailing lists I read this problema may be due to an
incorrect version of the extension, but it has been compiled with the
same sources version that phpinfo() reports (5.2.5).

I will appreciate any ideas of how to check now.