Postgresql query result PHP encode converted array to string

It's important to remember that JSON is a way of representing data inside a string. PHP does not know that something is JSON, only that it's a string.

You need to first decode the result from Postgres (which is a JSON string) so that you have a PHP array. Then you can encode that PHP array back to JSON:

$result = pg_query($connection, $queryString);
$resultArray = pg_fetch_all($result);
// $resultArray[0]['features'] is a string of JSON data from the DB
// For example, let's say it's '[1,2,3]'

// To you, this is obviously JSON: it looks like it,
// and you know you asked for that column to be JSON in the SQL.
// But PHP has no idea; it just sees a 7-character long string;
// so we need to tell it to decode that string:
$decoded_features_array = json_decode($resultArray[0]['features']);
// $decoded_features_array is now a PHP array containing 1, 2, and 3

// Obviously, you can just write that value straight back into the result
$resultArray[0]['features'] = json_decode($resultArray[0]['features']);

// Now the 'features' field of the result is an actual array,
// not a string, so we can do with it whatever we'd do with any other array

// That includes encoding it to send somewhere else - in this case, as JSON:
$json_result = json_encode($resultArray[0]);
// $json_result is now a string with all the fields
// and our PHP array gets encoded as a JSON array as we wanted:
// e.g. '{"type": "FeatureCollection", "features": [1,2,3]}'