- var_dump: documentation ( source)
- json_decode: documentation ( source)
- array_merge: documentation ( source)
- trim: documentation ( source)
<?php
$arr = [
0 => '"["Option 1", "Option 2", "Option 3"]"',
1 => '"["Option 1.1", "Option 1.2"]"'
];
foreach($arr as &$a) $a = json_decode(trim($a, '"'), true);
var_dump($arr);
var_dump([0=>array_merge(... $arr)]);