Function to recursively flatten multidimensional PHP array.

flatten.php

<?php
// Requires PHP 5.3+
// Found here: http://stackoverflow.com/a/1320156

function flatten_array(array $array) {
    $flattened_array = array();
    array_walk_recursive($array, function($a) use (&$flattened_array) { $flattened_array[] = $a; });
    return $flattened_array;
}

Nice. Here is also shorter version.

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

Bit longer. More php agnostic way.

function flattenArray($array)
{
static $flattened = [];
if(is_array($array) && count($array) > 0)
{   
    foreach ($array as $member) {
        if(!is_array($member)) 
        {
            $flattened[] = $member;
        } else
        {
            flattenArray($member);
        }
    }
}

return $flattened;
}

Or preserving keys by "smushing" them -- http://sandbox.onlinephpfunctions.com/code/8228ea592eb006903b29e87fa4cd7d55e3103dfc

function flattenWithKeys(array $array, $childPrefix = '.', $root = '', $result = array()) {
    // redundant with type hint
    //if(!is_array($array)) return $result;

    ### print_r(array(__LINE__, 'arr' => $array, 'prefix' => $childPrefix, 'root' => $root, 'result' => $result));

    foreach($array as $k => $v) {
        if(is_array($v) || is_object($v)) $result = flattenWithKeys( (array) $v, $childPrefix, $root . $k . $childPrefix, $result);
        else $result[ $root . $k ] = $v;
    }
    return $result;
}

Also works by typecasting objects or xml via (array) $obj

@zdenko:

You should include type hinting to avoid possible type errors. Slightly modified:

function flatten(array $arr) {
        return array_reduce($arr, function ($c, $a) { 
		return is_array($a) ? array_merge($c, flatten($a)) : array_merge($c, [$a]);

},
[]); 
    }

@Zenko:
If you pass arguments of a function (using func_get_args() which returns an array) to the function you run into Maxium memory exceeded error. How to fix please.