Cannot get AJAX answer to GET - php

Question

I'm trying to achieve the following:
when you click on H1, JQuery GETs info for certain key from the server and sends it back to user, where jquery is showing prompt with this info. PHP in server.php
$link = mysql_connect("mysql.hostinger.com.ua", "_my_login_", "_my_password_") or die("NOOO!");
mysql_select_db("u994953720_db");
$key;
if(isset($_GET['Key'])) {
$key = $_GET['Key'];
$selected = mysql_query("SELECT * FROM `Table` WHERE `Key`='".$key."'", $link);
$selected_row = mysql_fetch_array($selected);
if($selected_row!=null){
$response = array(
"result" => $selected_row['Value']
);
echo json_encode($response);
exit;
}else{
$response = array(
"result" => "Nope."
);
echo json_encode($response);
exit;
}
}
jQuery in the main page:
$('h1').on('click', function () {
$.ajax({
type: 'GET',
url: '/server/server.php',
data: {
'Key': 'Roman'
},
success: function(data) {
alert(data.result);
},
dataType: 'json'
});
});
But I don't have any effect. Could you guys show me how to fix my mistakes?
P.S.Working not working example
P.S.I am only starting learning PHP, so there can be some really stupid mistakes.
UPDATE: I've done everything like you guys said (I don't care about safety yet), but it still doesn't work. In the console there is the following message:
XHR finished loading: GET "http://site0.hol.es/server/server.php?Key=Roman".
When I include error method to ajax:
error: function(requestObject, error, errorThrown) {
alert("request objqect: " + requestObject + " . Error: " + error + " . Error thrown: " + errorThrown);
}
, there is an alert: parsing error, unexpected token <
UPDATE 1: I've understood, that my while page is being writed to json result, like here, but I don't have any other echo-s, how can it happen?
UPDATE: I've figured out, that PHP was sending the whole page what it was placed in, so, I simply removed that) But thank you for your comments about safety, I'll correct it

Answer 1

By looking at your site I can see that you are returning HTML + JSON. Try throwing an exit after your echo:
echo json_encode($response);
exit;
Also - you are not handling a no data state at all. You should be do something like:
if(isset($_GET['Key'])) {
// your valid request code
} else {
echo json_encode(array('result' => false, 'reason' => "You must send a Key"));
}
Furthermore you can debug you php code by hitting the endpoint directly and var_dumping/debugging the code that is run there. For starters, what is the output of $selected_row = mysql_fetch_array($selected); when you go to http://site0.hol.es/server/server.php?Key=Roman
Finally - just little warning, as others have mentioned. You are wide open to SQL injection attacks. Look into using PDO -
What PDO is:
http://php.net/manual/en/book.pdo.php
Why to use it:
http://code.tutsplus.com/tutorials/why-you-should-be-using-phps-pdo-for-database-access--net-12059
How to use it: http://wiki.hashphp.org/PDO_Tutorial_for_MySQL_Developers