[php] Converting PHP result array to JSON

I want to convert my result array to JSON format in PHP. Here is my code:

$row = mysql_fetch_array($result)

I want to convert $row to JSON format and pass the JSON data to an jQuery plugin.

$result = mysql_query($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);