I'm trying to convert the brackets to hyphens, but instead the date variable loses its value:
echo $date; // outputs 26/05/2015 10:41:56sd2
$date = date('Y-m-d H:i:s', $date);
echo $date; // outputs 969-12-31 18:00:26
The second parameter to date() must be a Unix timestamp. You're giving it a string.
That date format is invalid and won't work with strtotime() anyway. When you use / as the date separator US format is assumed. There is no 26th month.
The last three characters of that is not valid in any standard that I know of and will break any date function unless you specifically account for it (which you can't do with date() or strtotime())
Use DateTime::createFromFormat() to do this:
$date = DateTime::createFromFormat('d/m/Y H:i:s???', '26/05/2015 10:41:56sd2');
echo $date->format('Y-m-d H:i:s');