php to send json

<?php
$servername = "localhost";
$username   = "root";
$password   = "";
$database   = "userdata";
$connect    = mysql_connect($servername, $username, $password);
$check      = mysql_select_db($database, $connect);
if (!$check) {
    echo"not connected bro";
}
if ($_SERVER['REQUEST_METHOD'] == 'POST') {
   
    $name     = $_POST['name'];
   
    $username = $_POST['username'];
   
   
    $password = $_POST['password'];
   
   
    if ($name == '' || $username == '' || $password == '' ) {
        echo 'please fill all values';
    } else {
        $numrow = 0;
      
       
        if ($numrow > 0) {
            echo "values already exist";
        } else {
            $sql    = "INSERT INTO `uesr` (`id`, `username`, `password`) VALUES (NULL, '$username', '$password')";
           
            $result = mysql_query($sql);
            echo "successfully resgistered";
        }
    }
}
?>