<?php
function parseValuesFromKey($key, $values)
{
    $result = array();

    $keys = explode('.', $key);
    foreach($keys as $k)
    {
        if (array_key_exists($k, $values))
            $result[] = $values[$k];
    }

    return $result;
}

$someStr = '1.4.12.56';

$array = array (
      1 => "oranges",
      2 => "apples",
      3 => "pears"
  );

var_dump(parseValuesFromKey($someStr, $array));