$json = '[{"id":"1","category":"public health","type":"top"},{"id":"2","category":"environment","type":"top"},{"id":"3","category":"global unrest","type":"top"},{"id":"4","category":"military","type":"top"},{"id":"6","category":"super powers","type":"top"},{"id":"7","category":"technology","type":"top"},{"id":"8","category":"human rights","type":"top"},{"id":"60","category":"space race","type":"top"},{"id":"67","category":"globalism","type":"top"},{"id":"87","category":"government","type":"top"}]';
$array = json_decode($json, 1); // decode as an array
$column = 'category';
$categories = array_column($array, $column); // get the $column values
echo '<pre>' . print_r($categories, 1) . '</pre>';
gives
Array
(
[0] => public health
[1] => environment
[2] => global unrest
[3] => military
[4] => super powers
[5] => technology
[6] => human rights
[7] => space race
[8] => globalism
[9] => government
)