php字符串去掉中文

$a = "<a>河蟹社会afeowahgieawhjozf候蛇fawgwea维稳aghoiawghowi我日</a>"; 
$result = preg_replace('/([\x80-\xff]*)/i','',$a); 
var_dump($result);               		

<?php
try
{
    $hostname = "localhost";
    $dbname = "xxxx";
    $username = "root";
    $pw = "xxxx";
    $pdo = new PDO ("mysql:host=$hostname;dbname=$dbname", "$username", "$pw");
    $db->query("set names utf-8");
} 
catch (PDOException $e)
{
    echo "Failed to get DB handle: " . $e -> getMessage() . "\n";
    exit;
} 
$query = $pdo -> prepare("select id,store_image FROM store");
$query -> execute();

for($i = 0; $row = $query -> fetch(); $i++)
{
    $new_file = preg_replace('/([\x80-\xff]*)/i', '',$row['store_image']);
    $id=$row['id'];
    $sql="update store set store_image='$new_file' where id=$id";
    $pdo->exec($sql);    
} 

/*
$it = new RecursiveDirectoryIterator("./avatar");
foreach(new RecursiveIteratorIterator($it) as $file)
{
    echo $file . "\n";
    $new_file = preg_replace('/([\x80-\xff]*)/i', '', $file);
    rename($file, $new_file);
} 
*/