API: Receiving invalid JSONString when creating a new Contact using PHP

Hello,

I'm integrating Zoho into our inhouse application, but I'm experiencing some issues with posting to the API.

All of them being related to JSONString is invalid. And I don't seem to find a solution.

Is there an online JSONString validator?

My JSONString for creating a new contact:

"JSONString" => "{"customer_name":"9a52ef41-df2b-42e9-ad8f-383f03208878","billing_address":{"address":"Foo 21","street2":"","city":"Palma","state":"Palma","zip":"77010","country":"Spain","phone":"099 999 999 999"},"contact_persons":{"email":"john@example.com","phone":"099 999 999 999"}}"

And this is the code calling the API.

public function request($url, $method = 'GET', $data = null)
{
    $options = [
        'query' => [
            'authtoken' => $this->token,
            'organization_id' => $this->organization,
        ],
    ];

    if (! is_null($data)) {
        $options['headers'] = [
            'Content-Type' => 'application/x-www-form-urlencoded;charset=UTF-8',
        ];

        $options['form_params'] = [
            'JSONString' => json_encode($data),
        ];
    }
    
    try {
        $request = $this->client->request($method, $this->api_url.$url, $options);

        return json_decode($request->getBody()->getContents(), true);
    } catch (Exception $exception) {
        throw $exception;
    }
}