Function to recursively flatten multidimensional PHP array.

flatten.php

<?php

function flatten_array($arg) {
  return is_array($arg) ? array_reduce($arg, function ($c, $a) { return array_merge($c, flatten_array($a)); },[]) : [$arg];
}

flatten_assoc.php

<?php

# Flatten array while preserving keys.
# Use custom prefix and / or glue for returned array keys

function flatten_assoc_array($arg, $prefix = "", $glue = ".") {
  $reducer = function ($c, $a) use ($arg, $prefix, $glue) {
              $_prefix = preg_replace("/^\\{$glue}/", "", "{$prefix}{$glue}{$a}");
              return array_merge($c, flatten_assoc_array($arg[$a], $_prefix, $glue));
             };
  return is_array($arg) ? array_reduce(array_keys($arg), $reducer, []) : array_combine([$prefix],  [$arg]);
}