The function: ap_register_output_filter and ap_register_input_filter were failing to compile.  After some looking I found that the functions were passed 3 arguements, but looking at the prototype from Apache 2.0.40 in util_filter.h the functions required 4 arguements:

AP_DECLARE(ap_filter_rec_t *) ap_register_input_filter(const char *name, ap_in_filter_func filter_func, ap_init_filter_func filter_init, ap_filter_type ftype);

But the code was calling: 
ap_register_output_filter("PHP", php_output_filter, AP_FTYPE_RESOURCE);

So I added: ap_register_output_filter("PHP", php_output_filter, NULL,  AP_FTYPE_RESOURCE);