php:最少/最频繁的字符

function findLeastFrequentChar($str) {
    $charCount = array();
    $str = strtolower($str); // 将字符串转换为小写，以忽略大小写

    // 统计每个字符出现的次数
    for ($i = 0; $i < strlen($str); $i++) {
        $char = $str[$i];
        if (ctype_alpha($char)) { // 只考虑字母字符
            if (isset($charCount[$char])) {
                $charCount[$char]++;
            } else {
                $charCount[$char] = 1;
            }
        }
    }

    // 找到出现次数最少的字符
    $leastFrequentChar = null;
    $leastFrequentCount = PHP_INT_MAX;
    foreach ($charCount as $char => $count) {
        if ($count < $leastFrequentCount) {
            $leastFrequentChar = $char;
            $leastFrequentCount = $count;
        }
    }

    return $leastFrequentChar;
}

$str = "Hello World";
$leastFrequentChar = findLeastFrequentChar($str);
echo "The least frequent character in '$str' is '$leastFrequentChar'.";