<?php
declare(strict_types=1);
/**
*
* Pig Latin is a game of alterations played on the English language game.
* To create the Pig Latin form of an English word the initial consonant sound is transposed to the end of the word
* and an ay is affixed (Ex.: "banana" would yield anana-bay).
* Read Wikipedia for more information on rules.
*
*/
function pigLatin(string $str){

	if(!ctype_alpha($str)){
		throw new InvalidArgumentException('в строке могут быть только буквы');
	}

	$str = strtolower($str);

	define('VOWELS', array('a','e', 'i', 'o', 'u', 'y'));

	$i = 0;
	while($i<=strlen($str)-1){
		if(!in_array($str[$i],VOWELS)){
			$str .= $str[$i];
			$str = substr($str, 1);
		}else{
			break;
		}
	}

	return $str . "ay";
}

var_dump(pigLatin('three'));