While you could, perhaps, develop a way to generate a row number in a query, it should not be necessary to have a row number at query time. Many users have row ID values stored in their tables to assist with joins and where clauses. Others add row numbers at display time to help users distinguish rows, but PHP should not require row numbers to properly process data.
In any case, as MySQL does not have a row number meta-column, it would be better to add a row number in your PHP code while iterating rows by way of a counter or similar method.
This was first published in January 2006