<?php
 
$rawPhone = "+7(123 123-45 67";
$rawPhone = trim($rawPhone);
 
$regExpProper = '/^(?:8|\\+7)((?:[\\s\\(\\)\\-]*[0-9]){10})$/';
preg_match_all($regExpProper, $rawPhone, $matches);
 
if (empty($matches[0])) {
    echo "Ошибка при вводе телефона, попробуйте еще раз";
} else {
    $regExpClean = '/[^0-9\\+]/';
    $cleanPhone = preg_replace($regExpClean, "", $matches[1][0]);
    $properPhone = "8{$cleanPhone}";
    echo $properPhone;
}

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