php如何去除重复字

$text = '数组aabbccdd';
$text_filter = '';
$filter = array();
$len = mb_strlen($text, 'utf-8');
for ($i = 0; $i<$len; $i++) {
$char = mb_substr($text, $i, 1, 'utf-8');
if (!isset($filter[$char])) {
$text_filter .= $char;
$filter[$char] = $char;
}
}
echo $text_filter;