For Western Arabic numbers (0-9):
$words = preg_replace('/[0-9]+/', '', $words);
For all numerals including Western Arabic (e.g. Indian):
$words = '????';
$words = preg_replace('/\d+/u', '', $words);
var_dump($words); // string(0) ""
\d+
matches multiple numerals./u
enables unicode string treatment. This modifier is important, otherwise the numerals would not match.