Reaching deep into arrays using array_reduce in PHP

Recently saw an article on doing this in JavaScript, so figured I’d try it out in PHP.

The original article:

Safely Accessing Deeply Nested Values In JavaScript

The idea is you have an deeply nested array:

$x = [
  'a' => 1,
  'b' => [
    'c' => 2,
    'd' => [3, 4, 5],
  ],
];

You want to easily get the nested value at the path b/d/0.

The old-school way to do this without producing warnings or notices would be to do something like this:

if (
  array_key_exists('b', $x) &&
  is_array($x['b']) &&
  array_key_exists('d', $x['b']) &&
  is_array($x['b']['d']) &&
  array_key_exists(0, $x['b']['d']
) { return $x['b']['d'][0]; }

I know, pain in the butt, right? Let’s see if we can do that more easily. PHP has a function called array_reduce which will iterate over an array with a callback until it gets a null or there is nothing left to iterate, in which case it returns the result of the last call to the callback function.

PHP: array_reduce - Manual

In order to use this, we need to have a callback which will simply check to see if the value is an array and if a specific key exists.

function keyReduce(array $arr, $idx) {
  return (
    array_key_exists($idx, $arr)
  ) ? $arr[$idx] : null;
}

In the above, we’re just using a ternary to check and return a value if it is available, or null.

Now, we can plug this into array_reduce with our path split into an array [‘b’, ‘d’, 0] and our original array.

$result = array_reduce(['b', 'd', 0], 'keyReduce', $x);
echo $result;

We get our result:

3

What if we give it a path that doesn’t exist? We just get null back. No errors, no warnings, no notices.

We can also use this to get any section of the original array using a path. If we leave off the 0 from the path requested, we get that chunk.

$result = array_reduce(['b', 'd'], 'keyReduce', $x);
print_r($result);

We get…

Array
(
    [0] => 3,
    [1] => 4,
    [2] => 5
)

So there you go. An easy way to recurse through a deep array to fetch a value. You could also wrap the whole thing in a function to make it a bit easier.

function getArrayPath(array $path, array $deepArray) {
  $reduce = function(array $xs, $x) {
    return (
      array_key_exists($x, $xs)
    ) ? $xs[$x] : null;
  };
  return array_reduce($path, $reduce, $deepArray);
}
print_r(getArrayPath(['b','d', 0], $x));
3