poor_mysql.php

<?php

/**
 * Aggregates values as a JSON array
 * @param array $columns
 * @return string
 */
function json_object(array $columns): string
{
    reset($columns);

    $alias = key($columns);
    $column = array_shift($columns);

    $json = sprintf("CONCAT('{','\"%s\":\"',%s,", $alias, $column);

    foreach ($columns as $alias => $column) {
        $json .= sprintf("'\",\"%s\":\"',%s,", $alias, $column);
    }

    $json .= "'\"}')";

    return $json;
}

/**
 * Aggregates name/value pairs as a JSON object
 * @param array $columns
 * @param bool $distinct
 * @return string
 */
function json_object_agg(array $columns, bool $distinct = true): string
{
    return json_array(json_object($columns), $distinct);
}

/**
 * Aggregates values as a JSON array
 * @param string $column
 * @param bool $distinct
 * @return string
 */
function json_agg(string $column, bool $distinct = true): string
{
    return json_array("CONCAT('\"', $column, '\"')", $distinct);
}

/**
 * This is an internal function. Use json_agg/json_object_agg instead.
 * @param string $expr
 * @param bool $distinct
 * @return string
 */
function json_array(string $expr, bool $distinct = true): string
{
    return sprintf("CONCAT('[', COALESCE(GROUP_CONCAT(%s %s), ''), ']')", $distinct ? "DISTINCT" : "", $expr);
}

$pdo = ...
$query = 'select
  q.text,
  %s as answers
FROM questions q
  INNER JOIN answers a ON a.question_id = q.question_id
group by q.question_id';
$stmt = $pdo->prepare(sprintf($query, json_object_agg([
    "text" => "a.text",
    "id_correct" => "a.is_correct"
]));
...