下面是我的验证规则：

$validationCondition    = array(
                            'fname'     => 'required|min:2',
                            'lname'     => 'required|min:2',
                            'email'     => 'required|email|unique:users,email',
                            'isd'       => 'required|unique:users,isd',
                            'mobile'    => 'required',
                            'password'  => 'required'
                        );
$validationMessages     = array(
                            'fname.required'    => 'Please provide first name',
                            'fname.min'         => 'First name should be atleast 3 characters long',

                            'lname.required'    => 'Please provide last name',
                            'lname.min'         => 'Last name should be atleast 3 characters long',

                            'email.required'    => 'Please provide email',
                            'email.email'       => 'Please provide a valid email id',
                            'email.unique'      => 'This email id has already been registered. Choose any other valid email id.',

                            'isd.required'      => 'The city name should be atleast 3 characters long',

                            'mobile.required'   => 'Please provide mobile number',

                            'password.required' => 'Please provide password'
                        );

在用户表中，要检查isd和移动的以确保联系号码是否唯一。
怎么检查？我知道如何检查单个字段的唯一性，但不知道如何检查组合值是否唯一。