<?php
$regexp = '/^( ?8|\+ ?7)([ \-()]*[0-9]){10}$/';
$number = "+ 7 ( 812 ) 678 - 67 - 67";
echo $number . "\n";

if (preg_match($regexp, $number)) {
	$number = preg_replace("/[ \-()]/", '', $number);
}
if (preg_match("/\+7/", $number)) {
	$number = preg_replace("/\+7/", "8", $number);
}

echo $number;