Get values between two keys of an associative array

You pass in a key $from and a key $to. To have something from - to working, you need to know the order as well. You get the order of keys with array_keys:

$keys = array_flip(array_keys($array));

Now you can locate both the offset of $from and $to keys:

array_slice($array, $keys[$from], $keys[$to] - $keys[$from] + 1);

Compile as a full example:

<?php
/**
 * Most efficient way to get keys,values between two keys of an Associative array
 * @link http://stackoverflow.com/q/11358192/367456
 */

$array = array("one" => "one", "two" => "two", "three" => "three", "four" => "four");

$slice = function($from, $to) use ($array)
{
    $keys = array_flip(array_keys($array));
    if (isset($keys[$from]) and isset($keys[$to])) {
        return array_slice($array, $keys[$from], $keys[$to] - $keys[$from] + 1);
    }
    throw new InvalidArgumentException('Invalid from and/or to key.');
};

var_dump($slice('one', 'three'));

Output:

array(3) {
  ["one"]=>
  string(3) "one"
  ["two"]=>
  string(3) "two"
  ["three"]=>
  string(5) "three"
}

Assuming your input array has contiguous keys, why not just:

$newArray = array_slice($array, $key1, $key2 + 1);

EDIT

Oh wait, that will only work when $key1 = 0, try this instead:

$newArray = array_slice($array, $key1, ($key2 - $key1) + 1);

This still requires that $key1 < $key2, but from what you say I imagine it always will be.

ANOTHER EDIT

In order to accomplish this without looping the array (with would of course be the easiest way) you need to convert the string keys to numerics so they can be used with array_slice(). This works:

$var = array("one"=>"one","two"=>"two","three"=>"three","four"=>"four");
$key1 = 'one';
$key2 = 'three';

$keys = array_keys($var);
$key1index = array_search($key1, $keys);
$key2index = array_search($key2, $keys);

$newArray = array_slice($var, $key1index, ($key2index - $key1index) + 1, TRUE);

print_r($newArray);

In terms of time complexity, you can process the whole array in a single pass with just one nested condition block and a conditional break.

Obviously the following algorithm can easily be adjusted to compare values or return an indexed array if desired.

The the starting point is matched or the result array is not empty, push the current value into the result array. If ending point matches the current value, then stop looping.

Code: (Demo)

$array = [
    "one" => "one",
    "two" => "two",
    "three" => "three",
    "four" => "four"
];

$start = 'one';
$end = 'three';
$result = [];
foreach ($array as $k => $v) {
    if ($start === $k || $result) {
        $result[$k] = $v;
        if ($end === $k) {
            break;
        }
    }
}
var_export($result);