Answer by vess for Is it possible to force Excel recognize UTF-8 CSV files automatically?

In php you just prepend $bom to your $csv_string:

$bom = sprintf( "%c%c%c", 239, 187, 191); // EF BB BF
file_put_contents( $file_name, $bom . $csv_string );

Tested with MS Excel 2016, php 7.2.4