Recursively-Grouped Array Keys Multi-Dimensional Array

<?php
// Our Test Array - perhaps form-submitted data?
$array = array('phone' => array('zip'       => array(array('value' => 0,
                                                           'data' => 'integer',
                                                           'type' => 'text')),
                                'prefix'    => array(array('value' => 123,
                                                           'data' => 'integer',
                                                           'type' => 'text')),
                                'last4'     => array('newkey' => array(     // Added 'newkey' just for testing...
                            array('value' => 4567,
                                                           'data' => 'integer',
                                                           'type' => 'text')))),
                'first_name' => array('value' => 'John',
                                      'data' => 'string',
                                      'type' => 'text'),
                'last_name'  => array('value' => 'Doe',
                                      'data' => 'string',
                                      'type' => 'text'),
                'primary'    => array('value' => 1,
                                      'data' => 'bool',
                                      'type' => 'checkbox'));

/**
 * Flatten and Group Keys
 *
 * This iterates over an array (intended for multi-dimensional arrays) and returns an array
 * containing all of the keys of the array, grouped together.
 *
 * The purpose is for form fields with array NAME attributes and validating them while singling
 * out a particular form field as an invalid or valid field.
 *
 * This is part of my {@link https://github.com/amereservant/PHP-Form-Creator-Validator-Emailer-Class}
 * project and the major rewrite I'm currently working on...
 */
function flatten_keys($array)
{
    $cur = Array() ;
    if (is_array($array))
    {
        if (isset($array['value']))
            return null ;
        else
        {
            foreach ($array as $k => $v)
            {
                $fk = flatten_keys($v) ;
                if ($fk === null)
                    $cur[] = $k ;
                else if (is_array($fk))
                    foreach ($fk as $fk_k => $fk_v)
                        $cur[] = array_merge(array($k), is_array($fk_v) ? $fk_v : array($fk_v)) ;
            }
        }
    }
    else
        return false ;

    return $cur ;
}

// Test the function ...
echo '<pre>';
print_r(flatten_keys($array)) ;
echo '</pre>';