PHP: US Phone # validation

phone.php

<?php
		// simple phone number
		$phone_number_as_integer = preg_replace("/[^0-9]/", "", $phone);
		
		if( !preg_match("/^()?[0-9]{3}[0-9]{3}[0-9]{4}$/i", $phone_number_as_integer) ) 
		{
			$phone_error = 'Please enter a valid phone number.';
		}