实时数据更新php + mysql

I'm currently using long-polling method to update data in realtime using ajax and php loop:

function getdata(){
 //database request...
 if(new data){
 echo new data;
 }
 }else{ 
 getdata(); //loop again
 }
}

The problem is that this method requires a unique Apache process for every user and eats a lot of memory, so my site can't handle more than 100 users.

Is there any other more efficient method of getting realtime data from mysql database?

Edit: And I need to use additional php function file_get _contents() for every loop to check if data has been changed.

I think that if you will use a unique key fr every user than it can be solve.... For example just put a new num on each customer request and then fetch the last one by using DESC and than check to the new one.