How to print the array value inside the json array in php

i use the api to call the php page. this api have this json post data

{"product[]":["Layer Management System","Broiler Management System"]}

how can i get the data seprately for product[0]. i use this method

$data = json_decode(file_get_contents('php://input'), true);
print_r($data);
echo  $data["product[]"];

but it was not woking and show the error below

<b>Notice</b>: Array to string conversion in
<b>/production/test_reg3.php</b> on line <b>8</b><br />
Array

and one more method is

  $data = json_decode(file_get_contents('php://input'), true);
    print_r($data);
    echo  $data["product[0]"];

it shows the error below

<b>Notice</b>: Undefined index: product[0] in
<b>/production/test_reg3.php</b> on line <b>8</b><br />

i need to store product[] value in any variable

help me with the good result
thank your for spending your time for me

>Solution :

It should be

$data["product[]"][0]

Don’t get confused between the brackets which are part of the PHP syntax and the brackets which are part of the property name.