I works finally.
Because I use as2 before, not quite familer with as1, therefore I waste a lot of time to test.

Therefore I wrote the code which I tested here. I hope it can help someone like me, struggle with this tiny problem.
Flash :
loadVariables("http://127.0.0.1:1001/Basic/uldl?klCommand=loadVariables&klUrl=http://..test.php","GET");trace(yes);
Server side
<?php
$error = "I come from PHP";
echo "&yes=$error";if ($_POST["sendToPHP"] == "hello"){
echo = "&yes=$error";}
?>
But I still have problem with POST method,
because server side didn't get any data from Flash.
loadVariables("
http://127.0.0.1:1001/Basic/uldl?klCommand=loadVariables&klUrl=http://...test.php?%26sendToPHP=hello&klUrlDecodeOff=1","/main","POST");
klError =0, but why php didn't get it??
Who can help me about it?