SSE: unexpected MIME type error - php

Question

I am using a PHP file as a worker for a SSE connection. This is my PHP file:
<?php
header('Content-Type: text/event-stream');
header('Cache-Control: no-cache');
$mysqli = new mysqli(* data *);
$listings = $mysqli->query("SELECT listing_id, price, buyer_id FROM listings WHERE schedule_requested = '1' AND finished = '0'");
$to_check = array();
while ($li = $listings->fetch_array()) {
$to_check[] = array($li["listing_id"], $li["price"], $li["buyer_id"]);
}
while (true) {
$change = 0;
$listings_2 = $mysqli->query("SELECT listing_id, price, buyer_id FROM listings WHERE schedule_requested = '1' AND finished = '0'");
$different = array();
while ($li_2 = $listings_2->fetch_array()) {
foreach ($to_check as &$li_check) {
if ($li_check[0] === $li_2["listing_id"]) {
if (intval($li_2["price"]) !== intval($li_check[1])) {
$li_check = array($li_2["listing_id"], $li_2["price"], $li_2["buyer_id"]);
$different[] = $li_check;
$change = 1;
}
break;
}
}
}
if ($change === 1) {
echo "data: " . json_encode($different) . PHP_EOL;
echo PHP_EOL;
flush();
}
sleep(1);
}
?>
Even though there is the header that specifies the content-type, I receive an error when I try to use the file:
EventSource's response has a MIME type ("text/html") that is not "text/event-stream". Aborting the connection.
I don't receive that error, anyway, when I am using the following code for the same file (commenting the infinite while loop).
<?php
header('Content-Type: text/event-stream');
header('Cache-Control: no-cache');
$mysqli = new mysqli(* data *);
$listings = $mysqli->query("SELECT listing_id, price, buyer_id FROM listings WHERE schedule_requested = '1' AND finished = '0'");
$to_check = array();
while ($li = $listings->fetch_array()) {
$to_check[] = array($li["listing_id"], $li["price"], $li["buyer_id"]);
}
/*while (true) {
$change = 0;
$listings_2 = $mysqli->query("SELECT listing_id, price, buyer_id FROM listings WHERE schedule_requested = '1' AND finished = '0'");
$different = array();
while ($li_2 = $listings_2->fetch_array()) {
foreach ($to_check as &$li_check) {
if ($li_check[0] === $li_2["listing_id"]) {
if (intval($li_2["price"]) !== intval($li_check[1])) {
$li_check = array($li_2["listing_id"], $li_2["price"], $li_2["buyer_id"]);
$different[] = $li_check;
$change = 1;
}
break;
}
}
}
if ($change === 1) {
echo "data: " . json_encode($different) . PHP_EOL;
echo PHP_EOL;
flush();
}
sleep(1);
}*/
echo "data: " . json_encode($different) . PHP_EOL;
echo PHP_EOL;
flush();
?>
I can't understand why it happens.

Answer 1

The SSE standard clearly states that if the response does not contain text/event-stream, it MUST error the connection.
HTTP 200 OK responses that have a Content-Type other than text/event-stream (or some other supported type) must cause the user agent to fail the connection.
SSE protocol