Simply use str_replace:
$text = str_replace(' ', '_', $text);
You would do this after your previous substr
and strtolower
calls, like so:
$text = substr($text,0,10);
$text = strtolower($text);
$text = str_replace(' ', '_', $text);
If you want to get fancy, though, you can do it in one line:
$text = strtolower(str_replace(' ', '_', substr($text, 0, 10)));