{"data":["file1.txt","file2.txt","file3.txt"]}

public function getDirectories() {
    $dir = $this->getConfig(); // My Directory
    $array = array();
    $handle = opendir($dir);
    while (FALSE !== ($entry = readdir($handle))) {
        if ($entry == '.' || $entry == '..')
            continue;
        $array[] = $entry;
    }
    return $array;
}

class ArrayValue implements JsonSerializable {

    public function __construct(array $array) {
        $this->array = $array;
    }

    public function jsonSerialize() {
        return array('data' => $this->array);
    }
}

{"data":{"file1.txt","file2.txt","file3.txt"}}