Php mariadb unique field selection for index of query result.

Hi, Quick question. I may be missing something but for years I've been processing the output of a query into a new array using the unique(normally primary key) field in the table as the index so I can pull data matches simply further into the script without another loop through all the returned data. Is there an option I'm missing to assign a field as the output array index from a PHP based query? Thanks, -- Richard Scotford