If I generate a file and open the excel this is I always get
“excel cannot open the file because the extension is not valid. verify that the file is not been corrupted and that file extension matches the format of the file”
If you open the file using your programming editor, does it contain anything at all or does it contain any php errors?
Also, since excel can open a csv file, it is usually much simple to generate a csv of the output then trying to produce a valid excel file.
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