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)));
$string = "this is the test for string." ;
$string = str_replace(' ', '_', $string);
$string = substr($string,0,10);
var_dump($string);
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