Issue in creating API request of create an item in ZOHO inventory

Issue in creating API request of create an item in ZOHO inventory

I am using an API in which i faced an issue.

//CODE//

    $data = array(
        "unit"                  => "10",
        "item_type"             => "inventory",
        "product_type"          =>"goods",
        "is_taxable"            => "true",
        "description"           => "product description",
        "purchase_account_id"   => '123',
        "inventory_account_id"  => '1235',
        "name"                  => 'hibiscus',
        "rate"                  => '10',
);
    $data_string = json_encode($data);

    curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");
    curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query($data_string));
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, FALSE);
    curl_setopt($ch, CURLOPT_HTTPHEADER, array("Content-Type: application/x-www-form-urlencoded") );

//END CODE//

By this i received an erorr:
{"code":4,"message":"Invalid value passed for JSONString"}

Will you please let me know asap what i am doing wrong. I also searched about same issue but no luck.