<?php

// номер 

$number = "8 (911) - 123 - 45 - 67";
$regExp = "/^(\+ *7| *8)([ ()-]*\d){10}$/m";
$regExp1 = "/^(\+ *7| *8)/";
$regExp2 = "/ |-|\(|\)/";
if (preg_match($regExp, $number)) {
	$result = preg_replace($regExp1, "8" , $number);
	$result = preg_replace($regExp2, "" , $result);
}
echo $result;