$username = 'username';
$password = 'password';
$ldap_host = "domain.com";
$ldap_port = 389;
$base_dn = "DC=domain,DC=com";
$filter = '(sAMAccountName=' . $username . ')';
$connect = ldap_connect($ldap_host, $ldap_port) or exit("Error : Could not connect to LDAP server.");
if ($connect) {
    ldap_set_option($connect, LDAP_OPT_PROTOCOL_VERSION, 3);
    ldap_set_option($connect, LDAP_OPT_REFERRALS, 0);
    if (@$bind = ldap_bind($connect, "$username@domain.com", $password)) {
        echo "Bind Successfull";
    } else {
        echo "Invalid Username / Password";
    }
}