How to execute a CURL with POST method

I have a problem when trying to execute a CURL passing parameters with the POST method, the CURL that I want to execute is this:

curl -X POST \
  'https://api.mercadopago.com/v1/payments?access_token=ACCESS_TOKEN_ENV' \
  -d '{
    "token":"b3a7dbec3eb0d71798c4f19fec445795",
    "installments":1,
    "transaction_amount":58.80,
    "description":"Point Mini a maquininha que dá o dinheiro de suas vendas na hora",
    "payment_method_id":"visa",
    "payer":{
    "email":"[email protected]"

    }

These are the minimum data that Mercado Pago asks me to be able to make a record of a new payment, I am trying to execute this query from PHP, I am trying the following:

<?php

  $curl = curl_init();
  $token = "5ad1c4126a25bbabc068961e69e7d5ce";
  $installments = 1;
  $transaction_amount = 100;
  $description = "Pago desde cURL";
  $payment_method_id = "master";
  $payer = array(
  "email" => "[email protected]"
  );

  curl_setopt($curl, CURLOPT_URL,'https://api.mercadopago.com/v1/payments?access_token=TEST-1234-56789-101112-456789');
  curl_setopt($curl, CURLOPT_HEADER, 0);
  curl_setopt($curl, CURLOPT_HTTPHEADER, array('Content-Type: application/json'));
  curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
  curl_setopt($curl, CURLOPT_CUSTOMREQUEST, "POST");
  curl_setopt($curl, CURLOPT_FOLLOWLOCATION, 1);
  curl_setopt($curl, CURLOPT_POSTFIELDS, "{\"transaction_amount\" : \"$transaction_amount\",
                                           \"token\" : \"$token\",
                                           \"description\" : \"$description\",
                                           \"installments\" : \"$installments\",
                                           \"payment_method_id\" : \"$payment_method_id\",
                                           \"payer\" : \"$payer\"}");
  $data = curl_exec($curl);
  curl_close($curl);
  echo $data;

?>

But when executing in postman it gives me the following two errors:

<br />
<b>Notice</b>:  Array to string conversion in <b>C:\xampp20\htdocs\Apoya pagos\Curl\pago_curl.php</b> on line <b>24</b><br />
{
    "message": "transaction_amount attribute must be numeric",
    "error": "bad_request",
    "status": 400,
    "cause": [
        {
            "code": 4003,
            "description": "transaction_amount attribute must be numeric",
            "data": null
        }
    ]
}

I don't know what I'm doing wrong when sending my payer field as an array and I don't understand why it tells me that transaction_amount must be numeric when I'm passing it as a number.