// Export to CSV
if($_GET['action'] == ‘export’) {

$rsSearchResults = mysql_query($sql, $db) or die(mysql_error());

$out = ”;
$fields = mysql_list_fields(‘database’,'table’,$db);
$columns = mysql_num_fields($fields);

// Put the name of all fields
for ($i = 0; $i < $columns; $i++) {
$l=mysql_field_name($fields, $i);
$out .= ‘”‘.$l.’”,’;
}
$out .=”\n”;

// Add all values in the table
while ($l = mysql_fetch_array($rsSearchResults)) {
for ($i = 0; $i < $columns; $i++) {
$out .=’”‘.$l["$i"].’”,’;
}
$out .=”\n”;
}
// Output to browser with appropriate mime type, you choose
header(“Content-type: text/x-csv”);
//header(“Content-type: text/csv”);
//header(“Content-type: application/csv”);
header(“Content-Disposition: attachment; filename=search_results.csv”);
echo $out;
exit;
}

In some situations it might also be useful to force the user’s browser to download the file. Adding these lines should do the trick.

header("Cache-Control: must-revalidate, post-check=0, pre-check=0");
header("Content-Disposition: attachment; filename = $filename");
header("Content-Length: " . strlen($out));

Source: http://snipplr.com/view/2234/export-mysql-query-results-to-csv/