Stripping a phone number of its parenthesis, spaces, and hyphens in PHP?

With a regexp. Specifically, use the preg_replace function:

$phone = preg_replace('/\D+/', '', $phone);

preg_replace("/[^0-9]/", "", $phone);

Cumbersome method for regex avoiders:

implode(array_filter(str_split('(555) 555-5555'), 'is_numeric'));