Append Data to Json Array Issue

Append data to JSON array using PHP

$accountData is an object, as it should be. Array access is not valid:

array_push($accountData->loginHistory, $newLoginHistory);
// or simply
$accountData->loginHistory[] = $newLoginHistory;

Appending JSON array object to formdata

If your API accepts your data in that format, you cannot send it as JSON, you need to create a separate FormData entry for each of the lines you've mentioned.

For example

const contacts = [{ "Country":"country 1", "Phone":"123" },{ "Country":"country 2", "Phone":"456" }];


var formdata = new FormData();

// This could be made fancier but it explains how to fix the problem
for (let i=0; i < contacts.length; i++) {
   formdata.append(`Contacts[${i}].Country`, contacts[i].Country);
   formdata.append(`Contacts[${i}].Phone`, contacts[i].Phone);
}

Note that json-form-data does it for you

const data = {
  Contacts: [{
    "Country": "country 1",
    "Phone": "123"
  }, {
    "Country": "country 2",
    "Phone": "456"
  }]
};

const formData = jsonToFormData(data);
for (const [key,value] of formData.entries()) {
    console.log(`${key}: ${value}`);
}

<script src="https://unpkg.com/json-form-data@^1.7.0/dist/jsonToFormData.min.js"></script>

JSON='[{"name": "Jon", "class": "senior"}]'
echo "$JSON" | jq '. += [{"new_key": "new_value"}]'

[
  {
    "name": "Jon",
    "class": "senior"
  },
  {
    "new_key": "new_value"
  }
]