php怎么返回json数据

<?php
header('Content-Type: application/json');

$data = array(
    'name' => 'John Doe',
    'age' => 30,
    'city' => 'New York'
);

echo json_encode($data);
?>

<?php
header('Content-Type: application/json; charset=utf-8');

$data = array(
    'name' => 'John Doe',
    'age' => 30,
    'city' => 'New York'
);

echo json_encode($data, JSON_UNESCAPED_UNICODE);
?>

<?php
header('Content-Type: application/json; charset=utf-8');

$data = array(
    'name' => 'John Doe',
    'age' => 30,
    'city' => 'New York',
    'description' => "This is a\nmulti-line\ndescription."
);

echo json_encode($data, JSON_UNESCAPED_UNICODE | JSON_PRETTY_PRINT);
?>

<?php
header('Content-Type: application/json; charset=utf-8');

$data = array(
    'name' => 'John Doe',
    'age' => 30,
    'city' => 'New York'
);

if (empty($data)) {
    echo json_encode(array('error' => 'No data available'));
} else {
    echo json_encode($data);
}
?>