PHP - Using OR in IF statement with function

I am using OR in if statement While adding OR its not working. Follow the code :

if(!exists('vouchers','v_id',$generate_id) OR !exists('vouchers','hash',$generate_hash))
{
     //Doing something
}

But when I do only one function call its workes.

if(!exists('vouchers','v_id',$generate_id)){//WORK...}

Any helps to fix my code ?