Can't validate Slack dialog fields. response_url call always fails - php

Question

I am trying to build a slack dialog, triggered by a slash command. The dialog pops up correctly, and when the user submits the data, slack hits an endpoints on my server.
From that moment there are two possible outcomes:
Everything is good and I post a confirmation to the user
The data submitted by the user does not pass my app's validation and I need to let the user know.
Let's focus on #2 for a second:
I am getting a response_url that seems valid (https:\/\/hooks.slack.com\/app\/MY-APP-ID\/433197747012\/kQANkbvc3lIViVyLSJKR695z)
For testing, I'd like to simulate a validation error with one of my fields, so I do this in my endpoint:
$errors = [
'errors' => [
[
'name' => 'vendor_email',
'error' => 'sorry, I do not like this dude'
]
]
];
// define the curl request
$ch = curl_init();
// $decoded->response_url does contain the correct slack URL...
curl_setopt($ch, CURLOPT_URL, $decoded->response_url);
curl_setopt($ch, CURLOPT_HTTPHEADER, [
'Content-Type: application/json',
'Content-Type: application/x-www-form-urlencoded'
]);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_FOLLOWLOCATION, true);
// set the POST query parameters
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query($errors));
// execute curl request
$response = curl_exec($ch);
error_log(" -- response_url response: " . json_encode($response). "\n", 3, './runtime.log');
// close
curl_close($ch);
The response I am getting from hitting the response_url is this:
{\"ok\":false,\"error\":\"invalid_request_data\"}
What am I doing wrong?
****** EDIT ************
Even when not going the CURL route, and just doing this:
return json_encode($errors)
will just close the dialog after submission, and will not trigger any validation error.

Answer 1

The respond_url is not for replying to submissions (e.g. for validation errors), but for sending a message back to the user in the channel.
Once the user completes the dialog you will get a request from Slack. You need to directly respond to that request. You can respond either with an empty response if everything was ok - or with a list of errors for validation. The response must be in JSON and occur within 3 seconds.
To respond all you need to do is echo your error array in JSON. Also make sure to correctly set the header to JSON, like so:
header('Content-Type: application/json');
echo json_encode($errors);
If you have no errors just echo nothing to automatically send a HTTP 200 OK.
See also here in the documentation about how to correctly respond to a submission.