How to send data back in AJAX in response from PHP? - php

Question

I want to make dynamic select options based on database values.
I am getting id when user select an option, I get this id in ajax and then this id is passed in PHP. In PHP I run SQL query based on provided id. Now I want to send response back in AJAX and then I will append my next select option with the given response.
This is my AJAX, from this code I am passing selected_val in PHP.
`
$(document).ready(function() {
$(".sdpt").change(function(){
let deptid = $(this).val();
console.log(deptid);
$.ajax({
method: "POST",
url: "joins.php",
data: { selected_val: deptid },
success: function(response){
console.log(response);
}
});
});
});
`
In PHP, I am getting data (pro_name) from database and adding it in an array. I want to return this array in ajax and then I will use it there. Now it is return the whole PHP code in response. Please guide where I am doing mistake and what is the alternative.
`
if (isset($_POST['selected_val']) ) {
$value = $_POST['selected_val'];
$myq = new Database();
$result = $myq->sql("SELECT * FROM programs where dept_id=$value");
$arr = array();
while($row = mysqli_fetch_assoc($result)){
$arr[] = $row['pro_name'];
};
echo json_decode("Google");
}
`