<?php
$_star = array(
	'★★★★★' =>'star5',
	'★★★★☆' =>'star4',
	'★★★☆☆' =>'star3',
	'★★☆☆☆' =>'star2',
	'★☆☆☆☆' =>'star1',
	'☆☆☆☆☆' =>'star0',
);
 //取得した値を使い配列からキーを取得
while ($star_val = current($_star)) {
	if ($star_val == 'star3') echo key($_star);
	next($_star);
}
?>
//結果
★★★☆☆