如果用户确实填满了字段,它就不会显示输出

what code should i put so that the my output "your car cost" won't show
i've been trying to figure out what to put but failed.

<?php
// define variables and set to empty values
$costErr = $modelErr = "";
$cost = $model = "";

if ($_SERVER["REQUEST_METHOD"] == "POST") {

  if (empty($_POST["cost"])) {
    $costErr = "Cost is required";
  } else {
    $cost = test_input($_POST["cost"]);
  }

  if (empty($_POST["model"])) {
    $modelErr = "Model is required";
  } else {
    $model = test_input($_POST["model"]);
  }

}
function test_input($data) {
  $data = trim($data);
  $data = stripslashes($data);
  $data = htmlspecialchars($data);
  return $data;
}
?>

This is output when the user puts something on the field, but if the user won't enter a value on the field this output should not show up what should i do with this?

<?php
if (isset($_POST["age"]) && $_POST["age"] === "1 Year" && $_POST["conditon"] === "Yes") { 
  if ($_POST["age"] === "1 Year" && $_POST["conditon"] === "Yes") {
    echo "Your car Costs " .$cost;
    echo "<br>";
    echo "The model of your car is " .$model;
    echo "<br>";
    $price = $cost - ($cost * .20);
    echo "Your car costs in good condition " .$price;
  } else if($_POST["age"] === "1 Year" && $_POST["conditon"] === "No") {
    echo "Your car Costs " .$cost;
    echo "<br>";
    echo "The model of your car is " .$model;
    echo "<br>";
    $price = $cost - ($cost * .20) - ($cost * .10);
    echo "Your car costs in bad condition " .$price;
  }
}
?>