A minimalist GraphQL client for PHP

example.php

<?php

$query = <<<'GRAPHQL'
query GetUser($user: String!) {
   user (login: $user) {
    name
    email
    repositoriesContributedTo {
      totalCount
    }
  }
}
GRAPHQL;

graphql_query('https://api.github.com/graphql', $query, ['user' => 'dunglas'], 'my-oauth-token');

graphql-client.php

<?php

function graphql_query(string $endpoint, string $query, array $variables = [], ?string $token = null): array
{
    $headers = ['Content-Type: application/json', 'User-Agent: Dunglas\'s minimal GraphQL client'];
    if (null !== $token) {
        $headers[] = "Authorization: bearer $token";
    }

    if (false === $data = @file_get_contents($endpoint, false, stream_context_create([
        'http' => [
            'method' => 'POST',
            'header' => $headers,
            'content' => json_encode(['query' => $query, 'variables' => $variables]),
        ]
    ]))) {
        $error = error_get_last();
        throw new \ErrorException($error['message'], $error['type']);
    }

    return json_decode($data, true);
}

Somehow, I get this errors. Any help is GREATLY appreciated.

( ! ) Fatal error: Uncaught ErrorException: file_get_contents(xxxxx): Failed to open stream: HTTP request failed! HTTP/1.1 400 Bad Request in C:\wamp6433\www\research-portal\importAuthor.php on line 19

( ! ) ErrorException: file_get_contents(xxxxx): Failed to open stream: HTTP request failed! HTTP/1.1 400 Bad Request in C:\wamp6433\www\research-portal\importAuthor.php on line 19

This is my query that works just fine under the API Playground but not via code.

$query = <<<'GRAPHQL'
mutation { 
  createAuthor ( data: { username:"test", name:"test",email:"test@test.com", role:"Editor", client:"" } )  
  {  
    username
    name
    email
    role
    client   
  }
}
GRAPHQL;
graphql_query($endpoint, $query, array(), $accessToken);