Convert international phone +79004001234 to +7 900 400 12-34. Or local Russian 88004001234 to 8 800 400 12 34. Raw

pretty_phone.php

<?php
function pretty_phone($phone) {	
	if (!preg_match('/^(\+?)(\d)(\d{3})(\d{3})(\d{2})(\d{2})$/', $phone, $matches)) {
		return 'invalid phone';
	}

	$is_international = !empty($matches[0]) && $matches[0] === '+';
	$last_delimiter = $is_international ? '-' : ' ';
	$delimeters = [' ', ' ', $last_delimiter, $last_delimiter];

	$result = $matches[1] . $matches[2]; // +7 or 8
	for ($i = 3; $i <= 6; $i++) { 
		$result .= $delimeters[$i - 3] . $matches[$i];
	}

	return $result;
}