Online PHP editor | refs for 9PAT2

<?php function countUniqueCharacters(string $str): int { static $cache = []; return $cache[$str] ??= count( array_filter( count_chars($str, 1), fn($count) => $count === 1 ) ); } $tests = ['one', 'two', 'three', 'one', 'two']; foreach ($tests as $test) { echo PHP_EOL . "$test : " . countUniqueCharacters($test); }