Keeping an array sorted in PHP

Question

I have a PHP script which reads a large CSV and performs certain actions, but only if the "username" field is unique. The CSV is used in more than one script, so changing the input from the CSV to only contain unique usernames is not an option.

The very basic program flow (which I'm wondering about) goes like this:

$allUsernames = array();
while($row = fgetcsv($fp)) {
    $username = $row[0];
    if (in_array($username, $allUsernames)) continue;
    $allUsernames[] = $username;
    // process this row
}

Since this CSV could actually be quite large, it's that in_array bit which has got me thinking. The most ideal situation when searching through an array for a member is if it is already sorted, so how would you build up an array from scratch, keeping it in order? Once it is in order, would there be a more efficient way to search it than using in_array(), considering that it probably doesn't know the array is sorted?

Answer 1

Not keeping the array in order, but how about this kind of optimization? I'm guessing isset() for an array key should be faster than in_array() search.

$allUsernames = array();
while($row = fgetcsv($fp)) {
  $username = $row[0];

  if (isset($allUsernames[$username])) {
    continue;
  } else {
    $allUsernames[$username] = true;

    // do stuff
  }
}