gistfile1.php

<?php

function flatten($array, $prefix = '') {
    $result = array();
    foreach($array as $key=>$value) {
        if(is_array($value)) {
            $result = $result + flatten($value, $prefix . $key . '.');
        }
        else {
            $result[$prefix.$key] = $value;
        }
    }
    return $result;
}

gistfile2.php

<?php
 
$data = array(
    'user' => array(
        'email'   => 'user@example.com',
        'name'    => 'Super User',
        'address' => array(
            'billing' => 'Street 1',
            'delivery' => 'Street 2'
        )
    ),
    'post' => 'Hello, World!'
);
 
print_r(flatten($data));

gistfile3.txt

Array
(
    [user.email] => user@example.com
    [user.name] => Super User
    [user.address.billing] => Street 1
    [user.address.delivery] => Street 2
    [post] => Hello, World!
)