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.

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