<?php

error_reporting(-1);
header("Content-Type: text/plain; charset=utf-8");

$input = '8(911)596-55-55';
$regexp = '/^8[(\s]{1}[0-9]{3}[)\s]{1}[0-9]{3}[-\s]{1}[0-9]{2}[-\s]{1}[0-9]{2}$/';
$matches = array();

if (preg_match($regexp, $input, $matches)) {
    echo "Все верно. \n";
    var_dump($matches);
} else {
    echo "Не верно. \n";
}