replace non alpha numeric character

this will add -
<?php

$num="(480) 570-3705";
$num=preg_replace('/[^a-zA-Z0-9]+/', '-', $num);
echo $num;

?>

Only Numeric
$num=preg_replace('/[^0-9]+/', '', $num);

this will simply remove it
<?php

$num="(480) 570-3705";
$num=preg_replace('/[^a-zA-Z0-9]+/', '', $num);
echo $num;

?>