<? print("&your_flash_var1= $your_php_var1&your_flash_var2= $your_php_var2"); ?> and so on
you can also use this:
<?
$desired_text = "whatever";
$send_desired_text = "&flash_var=";
$send_desired_text .= rawurlencode($desired_text);
echo "$send_desired_text";
?>