<?php

$array = array(1 => 'foo', '4' => 'bar');

$firstIndexToEnsure = 1;
$lastIndexToEnsure = 5;
$defaults = array_fill($firstIndexToEnsure,
                       $lastIndexToEnsure - $firstIndexToEnsure + 1,
                       '');
$array += $defaults;

print_r($array);