$array = array('bar' => array('color' => 'green', 'size' => 'M'));

function set_value(&$root, $compositeKey, $value) {
    $keys = explode('.', $compositeKey);
    while(count($keys) > 1) {
        $key = array_shift($keys);
        if(!isset($root[$key])) {
            $root[$key] = array();
            $root = &$root[$key];
        }
    }

    $key = reset($keys);
    $root[$key] = $value;
}

echo "Original array:";
print_r($array);
set_value($array, 'bar.color', 'black');

echo "Final array:";
print_r($array);



// OUTPUT:
Original array:Array
(
    [bar] => Array
        (
            [color] => green
            [size] => M
        )

)
Final array:Array
(
    [bar] => Array
        (
            [color] => green
            [size] => M
        )

    [color] => black
)