can any body explain what is the use of csv files in php. i know it is used for export db as a xls files . in excel we can change the extension as .csv.

Php has some specific commands (fgetcsv fputcsv) to read and write csv files. If you have spreadsheet data that you want to upload, it is somewhat simpler to upload and read it as CSV than it would be to read it as an XLS file. You can also create a CSV file from PHPthat you can then read into Excel.

csv is a good flat file system for porting any kind of data between incompatible system