PHP-ext devel: How to call a userland function from within a dynamic

Hi all,

I've just finished my first dynamically-loadable-cpp-implemented PHP
extension and now I need to know how to call a userland PHP function
from inside the extension. I need to use Headers() to be able to
download a file that is generated by my extension. So far I've used this
function from the php-script that uses my extension, but I'd like to
embed this functionallity into it.

I've searched the php-sources to see the implementation of headers(),
but the function that seems to do the job (at SAPI.h):

SAPI_API int sapi_header_op(
sapi_header_op_enum op,
void *arg
TSRMLS_DC);

....fails at link time complaining about an undefined symbol related to
sapi_header_op_enum (which is also declared in that header). Do I need a
SAPI *.so or *.a module?

I apologyze if this is not the right newsgroup, if it isn't, please
point me to the correct one.

Thanks in advance.