The current documentation at php.net lists the following 
functions for the NIS/YP function set as:

yp_get_default_domain 
yp_order 
yp_master
yp_match 
yp_first
yp_next 

However in /ext/yp/php_yp.h the following addtional 
functions have beed defined and there is code in yp.c to 
make them work (atm I don't know if they work...).  
Descriptions from source code accompany function names

yp_all - Traverse map and call a function on each enrty
yp_cat - Return an array containing the entire map
yp_errno - Return the error code from the last call or 0 
if no error occured
yp_err_string - Returns corresponding error string fro the 
given code

Can you please add thess functions to the documentation 
and maybe add an example or two?  I'll be working on some 
yp stuff soon with php, so I'd be happy to send in some 
examples when I have them.