<?php
class Foo implements JsonSerializable {
    private $data = 'Bar';
    public function jsonSerialize() {
        return array('data'=>$this->data);
    }
}
echo json_encode(new Foo); // Outputs: {"data":"Bar"}

<?php
$a = array(1,2,3,array(4,5,6));
echo json_encode($a,JSON_PRETTY_PRINT);

[
    1,
    2,
    3,
    [
        4,
        5,
        6
    ]
]