<?php

$rawPhone = "моб. 8(123) 123-4 5 6-7 без выходных";
$regExp = '/(8|\\+7)([^0-9]*[0-9][^0-9]*[0-9][^0-9]*[0-9][^0-9]*)([^0-9]*[0-9][^0-9]*[0-9][^0-9]*[0-9][^0-9]*[0-9][^0-9]*[0-9][^0-9]*[0-9][^0-9]*[0-9])/';
$matches = array();
preg_match_all($regExp, $rawPhone, $matches);

if (empty($matches)) {
    die("Номер телефона не обнаружен");
}

$cleanPhone = "8{$matches[2][0]}{$matches[3][0]}";

$regExpNums = '/[^0-9]/';
$properPhone = preg_replace($regExpNums, "", $cleanPhone);
echo $properPhone;