Here's the shortest and the fastest way I found to convert a CSV file to MS Excel format. It supports string and numeric fields. I hope this function can avoid you using huge Excel PHP classes which are too complicated and slow (and require reading a lot of documentation) for such a basic task.

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/* Written by Alexis Bezverkhyy <alexis@grapsus.net> in September 2010
 * This is free and unencumbered software released into the public domain.
 * For more information, please refer to <http://unlicense.org/> */
 
/** Convert a CSV file to MS Excel format
 * 
 * @param string $in input file
 * @param string $out output
 * @param string $glue CSV glue
 * @param string $enclosure CSV enclosure character
 */
function csv2xls($in, $out, $glue=";", $enclosure='"')
{
        $fp_in = fopen($in, "r");
        $fp_out = fopen($out, "w");
        
        /* write Excel BOF */
        fputs($fp_out, pack("ssssss", 0x809, 0x8, 0x0, 0x10, 0x0, 0x0));
        
        /* Read CSV fields */
        for($row = 0; $fields = fgetcsv($fp_in, 0, $glue, $enclosure); $row++)
        {
                foreach($fields as $col=>$value)
                {
                        $value = trim($value);
                        $value = utf8_decode($value);
                        
                        /* string cell */
                        if(!is_numeric($value))
                        {
                                $l = strlen($value);
                                fputs($fp_out,
                                        pack("ssssss", 0x204, 8 + $l, $row, $col, 0x0, $l).$value);
                        }
                        /* numeric cell */
                        else 
                        {
                                fputs($fp_out,
                                        pack("sssss", 0x203, 14, $row, $col, 0x0).pack("d", $value));
                        }
                }
        }
        
        /* write Excel EOF */
        fputs($fp_out, pack("ss", 0x0A, 0x00));
        
        fclose($fp_out);
        fclose($fp_in);
}