- array_map: documentation ( source)
- json_encode: documentation ( source)
<?php
$arr = [];
$arr[2] = ['error_code' => 'a'];
$arr[4] = ['error_code' => 'b'];
$result = [];
$result['bad_lines'] = (object)array_map(function($line) { return $line['error_code']; }, $arr);
echo json_encode($result);