-System that used:
CentOS 6.2, squid v3.1x, LDAP Server, Linux DHCP server, httpd/apache server for wpad.dat

-Make sure squid can talk to LDAP server
# /usr/lib64/squid/squid_ldap_auth -b "dc=tahubachem,dc=local" -f "uid=%s" -h 192.168.10.6
Username password (Hit Enter)
(Result)

-Edit squid.conf

# OPTIONS FOR AUTHENTICATION
auth_param basic program /usr/lib64/squid/squid_ldap_auth -b "dc=tahubachem,dc=local" -f "uid=%s" -h 192.168.10.6
auth_param basic children 5
auth_param basic realm Squid proxy-caching web server
auth_param basic credentialsttl 2 hour
authenticate_ip_ttl 2 minutes

#using ldap auth
acl ldapauth proxy_auth REQUIRED

#one ip = one user login
acl one_ip_access max_user_ip -s 1

http_access deny one_ip_access
http_access allow ldapauth

#Wpad.dat will tell web browser the proxy address & port
http_port 3128

or see squid.conf here

-Make file wpad.dat in web server directory
# touch /var/www/html/wpad.dat
# vi /var/www/html/wpad.dat

function FindProxyForURL(url, host) {

if (shExpMatch(host, "127.*")
|| shExpMatch(host, "localhost")
|| shExpMatch(url, "*.tahubachem.net/*")
|| isPlainHostName(host)
|| dnsDomainIs(host, ".tahubachem.net")){
return "DIRECT";
}else{
return "PROXY 192.168.10.1:3128; DIRECT;";
// return "PROXY 123.123.123.123:8080; DIRECT;";
}
}

 
-Edit dhcpd.conf
Add new:

option wpad-url code 252 = text;
option wpad-url "http://192.168.10.6/wpad.dat\n";

or see dhcpd.conf here

-Testing:
PC using login staff1

Another PC using same login staff1 ,result -> Access Denied:

 
References:
http://www.findproxyforurl.com/index.html
http://www.cyberciti.biz/tips/howto-configure-squid-ldap-authentication.html
http://malforge.com/node/10