UDP socket_read() problem

Hello, I created an UDP socket using $sock=socket_create(AF_INET,SOCK_DGRAM,SOL_UDP) successfully and then used socket_bind($sock,$address,$port), up to there, no problems, but when I try to $buff=socket_read($sock,3) it just stays there doing nothing forever. What could be the problem? What is the normal behaviour of socket_read() when the socket is empty? I write to the socket (address/port) like crazy from the other side of the LAN, but nothing seems to be happening. Thanks, Martin