$client = new \GuzzleHttp\Client();
$url = 'https://flashpanel.io/api/servers/1/backups';
$response = $client->post(
    $url,
    [
        'headers' => [
            'Authorization' => 'Bearer {{AUTH_TOKEN}}',
            'X-Code' => '{{AUTH_CODE}}',
            'Content-Type' => 'multipart/form-data',
            'Accept' => 'application/json',
        ],
        'multipart' => [
            [
                'name' => 'code',
                'contents' => 'oauth_code_123'
            ],
            [
                'name' => 'drive',
                'contents' => 'drive'
            ],
            [
                'name' => 'host',
                'contents' => 'backup.example.com'
            ],
            [
                'name' => 'user',
                'contents' => 'backupuser'
            ],
            [
                'name' => 'port',
                'contents' => '22'
            ],
            [
                'name' => 'auth_password',
                'contents' => '1'
            ],
            [
                'name' => 'password',
                'contents' => 'secretpass'
            ],
            [
                'name' => 'passphrase',
                'contents' => 'myPassphrase'
            ],
            [
                'name' => 's3_provider',
                'contents' => 'AWS'
            ],
            [
                'name' => 's3_access_key_id',
                'contents' => 'AKIAIOSFODNN7EXAMPLE'
            ],
            [
                'name' => 's3_secret_access_key',
                'contents' => 'wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY'
            ],
            [
                'name' => 's3_region',
                'contents' => 'us-east-1'
            ],
            [
                'name' => 's3_endpoint',
                'contents' => 'https://s3.us-west-2.amazonaws.com'
            ],
            [
                'name' => 'memo',
                'contents' => 'Daily backup'
            ],
            [
                'name' => 'expression',
                'contents' => '0 0 * * *'
            ],
            [
                'name' => 'cycle',
                'contents' => '7'
            ],
            [
                'name' => 'all_sites',
                'contents' => '1'
            ],
            [
                'name' => 'sites[][id]',
                'contents' => '1'
            ],
            [
                'name' => 'sites[][excludes][]',
                'contents' => 'architecto'
            ],
            [
                'name' => 'excludes[]',
                'contents' => 'architecto'
            ],
            [
                'name' => 'excludes[]',
                'contents' => 'node_modules'
            ],
            [
                'name' => 'all_databases',
                'contents' => '1'
            ],
            [
                'name' => 'databases[]',
                'contents' => '1'
            ],
            [
                'name' => 'private_key',
                'contents' => fopen('/private/var/folders/qq/kvhmbqts2kjdk_q1d18jdvg80000gn/T/phpvtr59q28njv97OwiyI5', 'r')
            ],
        ],
    ]
);
$body = $response->getBody();
print_r(json_decode((string) $body));