Function to check number in mysql not working

Why not using AUTO_INCREMENT column instead trying to generate "random" id? Also, EventId function is not safe as choosing a id and storing it in database is not atomic so another process may already used same id.

If table has event_id column created as INT AUTO_INCREMENT SQL should look like this:

INSERT INTO foo (event_id, ...) VALUES (<everything EXCEPT event_id>);
event_id = SELECT LAST_INSERT_ID();

You don't state why it's not working, but my guess is that in //insert into events table code below your are using the $newEventId in the INSERT, and it works fine if the initial $newEventId causes (mysql_num_rows(mysql_query("SELECT id FROM events WHERE EventID=$gen" )) == 0) to return true, but not if it returns true on the recursive function.

Try returning the value on the break and using that instead of $newEventId

function EventId($gen)
{

  if (mysql_num_rows(mysql_query("SELECT id FROM events WHERE EventID=$gen" )) == 0) {
  return $gen; // return the number that will be set as $EventId and used in the insert
  break;
  }
  // recall function
  else 
  {
    $newEventId = rand(1000, 10000);
    EventId($newEventId);
  } 
}

$newEventId = rand(1000, 10000);
$EventId = EventId($newEventId);  // set the returned $gen to $EventId to be used in the Insert query

//insert into events table
mysql_query("INSERT INTO events ... EventID=$EventId..");