Convert Array to Json format using PHP

I have an array:

$arr = array('a' => 1, 'b' => 2, 'c' => 3, 'd' => 4, 'e' => 5);

// show the format
echo json_encode($arr);
?>

the return will be:

{"a":1,"b":2,"c":3,"d":4,"e":5}