<?php
include 'cn.php';
$query="SELECT SUM(monto) as `total` FROM tabla1 WHERE fecha BETWEEN '2019-01-00' AND '2019-01-31'";
$res=mysqli_query($conexion,$query);
$data=mysqli_fetch_array($res);
echo number_format($data['total'],2)
?>
$query="SELECT SUM(monto) as `total` FROM tabla1 WHERE YEAR(fecha)=2019 AND MONTH(fecha)=1";