CSV spreadsheet files with embedded functions

Let’s see how we can embed functions into a CSV file for later use in a spreadsheet program, notably Excel, Excel online and Google sheets.
The accompanying PHP code just display the CSV line (so can be captured to the clipboard on windows or to a file) with some random data and a function at the end.
The end of the CSV also has a type 109 subtotals function so you can filter data and see the results recalculated.
Other things in the code show how to convert an X coordinate (e.g. 3) into the spreadsheet column letter (e.g. 3=C). Useful when creating your own data.
Last thing demonstrate4d in the use of control and down-arrow to move to the end of a block of cells, control down-arrow to move to the right of a block of cells and adding shift to these keystrokes to select the data.
The repo for the code is at https://github.com/saaatlarge/spreadf...