How to convert mysqli result to JSON?

I have a mysqli query which I need to format as JSON for a mobile application.

I have managed to produce an XML document for the query results, however I am looking for something more lightweight. (See below for my current XML code)

$mysql = new mysqli(DB_SERVER,DB_USER,DB_PASSWORD,DB_NAME) or die(‘There was a problem connecting to the database’);

$stmt = $mysql->prepare(‘SELECT DISTINCT title FROM sections ORDER BY title ASC’);
$stmt->execute();
$stmt->bind_result($title);

// create xml format
$doc = new DomDocument(‘1.0’);

// create root node
$root = $doc->createElement(‘xml’);
$root = $doc->appendChild($root);

// add node for each row
while($row = $stmt->fetch()) :

$occ = $doc->createElement(‘data’);
$occ = $root->appendChild($occ);

$child = $doc->createElement(‘section’);
$child = $occ->appendChild($child);
$value = $doc->createTextNode($title);
$value = $child->appendChild($value);

endwhile;