adding record value from a function

i have a php function that generates random values,

i mean something like:

echo create_transactionid();// Returns for example a7YmTwG4

i want to use it to add values to the field transaction ID of my table transactions,

every time a value for the record transaction ID is added has to be done with the value generated with my own function automatically,

how can i do that and where i have to include that function

thanks a lot