<?php
 
error_reporting(-1);
 
$number = '+7 (905) 821-35-21'; // пользователь прислал номер
$number = preg_match('/^(\+\s?7|8)([-\s()]*\d){10}$/', $number, $matches); // проверяем корректность
$number = preg_replace('/[-\s()]*/', '', $matches[0]); // очищаем от тире, пробелов и скобок
$number = preg_replace('/\+7/', '8', $number); // заменяем +7 на 8
 
echo $number;

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