php 如何限制密码长度

<form action="" method="get">
pass: <input type="password" name="pass"> 限制六位
<input type="submit" value="提交">
</form>
<?php
$a=isset($_GET["pass"])?$_GET["pass"]:'';
if ($a!=''){
if (strlen($a)<=6){
echo "<script>alert('符合要求') </script>";
}else {
echo "<script>alert('不符合要求')</script>";
}
}
?>

if (strlen($pass)>10) exit('密码太长！');

<input type="password" name="pass" size="8" maxlength="6" />