The opposite of what Eugene Murai wrote in a previous comment is true when importing/uploading a file. For instance, if you export an Excel spreadsheet using the Save As Unicode Text option, you can use the following to convert it to UTF-8 after uploading:
//Convert file to UTF-8 in case Windows mucked it up
$file = explode( "\n", mb_convert_encoding( trim( file_get_contents( $_FILES['file']['tmp_name'] ) ), 'UTF-8', 'UTF-16' ) );