Converting PHP result array to JSON

Question

json_encode is available in php > 5.2.0:

echojson_encode($row);

$result = mysql_query($query) or die("Data not found."); 
$rows=array(); 
while($r=mysql_fetch_assoc($result))
{ 
$rows[]=$r;
}
header("Content-type:application/json"); 
echo json_encode($rows);