It depends.
This is probably the best answer to give you.
Do whatever you like, as long as it doesn't magically corrupt the data between write and read.
Lemme take a moment to explore a few options on your behalf.
- var_export() - might not be the best idea, securitywise
- serialize() - seems straightforward
- implode() - seems straightforward
- json_encode() - seems straightforward
There are probably other, more obscure, options available. You could even build up complex data sets with XML, if you like.
Alternatively, why not normalize the schema and add a new table to present that collections of array-data ? Normalization tends to save your bacon in the future.