Returning JSON from a PHP Script

I want to return JSON from a PHP script.

Do I just echo the result? Do I have to set the Content-Type header?

While you're usually fine without it, you can and should set the Content-Type header:

<?php
$data = /** whatever you're serializing **/;
header('Content-Type: application/json; charset=utf-8');
echo json_encode($data);

If I'm not using a particular framework, I usually allow some request params to modify the output behavior. It can be useful, generally for quick troubleshooting, to not send a header, or sometimes print_r the data payload to eyeball it (though in most cases, it shouldn't be necessary).

A complete piece of nice and clear PHP code returning JSON is:

$option = $_GET['option'];

if ( $option == 1 ) {
    $data = [ 'a', 'b', 'c' ];
    // will encode to JSON array: ["a","b","c"]
    // accessed as example in JavaScript like: result[1] (returns "b")
} else {
    $data = [ 'name' => 'God', 'age' => -1 ];
    // will encode to JSON object: {"name":"God","age":-1}  
    // accessed as example in JavaScript like: result.name or result['name'] (returns "God")
}

header('Content-type: application/json');
echo json_encode( $data );