<?php
 
$rawPhone = "+7 (911) 911-9-009";
 
$regExpClean = '/[^0-9\\+]/';
$cleanPhone = preg_replace($regExpClean, "", $rawPhone);
 
$regExpProper = '/(?:8|\\+7)((?:[^0-9]*[0-9]){10})/';
preg_match_all($regExpProper, $cleanPhone, $matches);
 
if (strlen($matches[1][0]) != 10) {
    echo "Не удается распознать телефонный номер";
} else {
    $phoneNumber = "8{$matches[1][0]}";
    echo "Распознан телефонный номер: $phoneNumber";
}

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