二阶段项目不会的知识点

<?php
$uid = $_POST["uid"];
$pwd1 = $_POST["pwd1"];
$pwd2 = $_POST["pwd2"];
include("./DBDA.class.php");
$db = new DBDA();
$sql = "select username from username";
$attr = $db->Query($sql);
foreach($attr as $v)
{    
    $attr1[]=$v[0];
}
$result = array_search($uid, $attr1);//返回结果是flase或者true

if ($result == false) 
{
    echo "没有重复";
} 
else//也就是$result==true
{
    echo "有重复";
}