using a variable as values in an array

I am trying to build an array from the results of a query on a mySQL table. I am using a loop to create this string in a variable $mystring: '30'=>true, '20'=>true I then need to use it like this: $defaultValues['ib_article'] = array($mystring); It isn't working. I am sure there is a better method, can anyone point me in the right direction? thanks charles