Array-like object with a default value that is populated when nonexistent keys are accessed. This is particularly useful to avoid constructs like this:

$counts = array();
foreach ($fruit as $fruit) {
  if (!isset($counts[$fruit])) {
    $counts[$fruit] = 0;
  }
  $counts[$fruit] += 1;
}

Instead, you can use PhutilArrayWithDefaultValue:

$counts = new PhutilArrayWithDefaultValue();
foreach ($fruits as $fruit) {
  $counts[$fruit] += 1;
}

Normally, the default value is 0 to allow the object to be used to count things or build bitmasks. You can change the default with setDefaultValue().