How to explode an array_agg

I have a data:

$data = '{123, 456, 231, 478, 123, 673}'; //This is a string. I used "array_agg" in SQL.

$value = explode("," , $data);

The problem with this is that I obtain something like this:

$value[0] = '{123';
$value[1] = '456';
.
.
$value[5] = '673}';

How can I get the exact value I want? I want to remove the '{}'. Thanks!