php 过滤重复字符串

<?php
function filterDuplicateChars($str) {
    $result = '';
    $seen = [];
    for ($i = 0; $i < strlen($str); $i++) {
        $char = $str[$i];
        if (!isset($seen[$char])) {
            $result .= $char;
            $seen[$char] = true;
        }
    }
    return $result;
}

$input = "hello world";
$output = filterDuplicateChars($input);
echo $output; // 输出 "helo wrd"
?>

<?php
function normalizeString($str) {
    return preg_replace('/[\x00-\x1F\x7F]/', '', $str);
}

$input = "hello\x07 world";
$normalizedInput = normalizeString($input);
$output = filterDuplicateChars($normalizedInput);
echo $output; // 输出 "helo wrd"
?>

<?php
function filterDuplicateCharsOptimized($str) {
    $result = '';
    $seen = [];
    $length = strlen($str);
    for ($i = 0; $i < $length; $i++) {
        $char = $str[$i];
        if (!isset($seen[$char])) {
            $result .= $char;
            $seen[$char] = true;
        }
    }
    return $result;
}

$input = "hello world";
$output = filterDuplicateCharsOptimized($input);
echo $output; // 输出 "helo wrd"
?>