I've designed a php website and i used simple CSV file as my database because of few reasons

1. my client had to pay more to active MySQL
2. database was a simple product list
3. it was easy to convert product list from existing Excel file to CSV
4. it is easy to make any changes

so i have few files to convert to CSV if i wanna update the website.

I know it's less secure specially if it's an online business (which is not).

question is : what are disadvantages?
is that slower than MySQL?
is there more possibility of occurring bug?
is that less professional?
or ...

"Flat-file" Databases were used for a long time by lots of people to do lots of things. They work.
For a small application you won't have much trouble and the speed won't be an issue. Flat file access is 'sequential', meaning you need to read from the start of the file to find the 'data you are looking for , whereas Databases are 'indexed', so read access starts at the data record you have the index for.
As long as the product lists are not too large, or the requests for data are small, you will be okay.
There are several Packages out there that might fill your needs. Check out Mylittlecms and Cutenews. Both of them run on Flat files, and might help you out. Check out their coding to see how they do stuff.
As the site gets larger, and busier, you might need to bite the bullet and convert to a Database. On the otherhand, you might actually enjoy all the coding that you will need to do.

jlhaslip, Thank you very much, i'm going to have a look Mylittlecms and Cutenews. now i got the bigger image i wanted.
Cheeeers