页面间的值传递有关问题。使用$_POST[]，但是还是无法传递

页面间的值传递问题。使用$_POST[]，但是还是无法传递
刚开始学这个，不知道问题出在哪里，请高人指点！
代码如下

HTML code

<!--

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-->
...

<form action = "results.php" method="post">
  Choose Search Type:<br />
  <select name ="searchtype">
    <option value="author">Author</option>
    <option value="title">Title</option>
    <option value="isbn">ISBN</option>
  </select>
  <br />
  Enter Search Term:<br />
  <input name="searchterm" type="text" size="40">
  <br />
  <input type="submit" name="submit" value="search" onClick="window.location='results.php'" />
</form>

...

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PHP code

<!--

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http://www.CodeHighlighter.com/

-->
...
<?php
  $searchtype=$_POST['searchtype'];
  $searchterm=trim($_POST['searchterm']);
  if(!$searchtype || !$searchterm) {
    echo 'You have not entered search details. Please go back and try again.';
    exit;
  }

...

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HTML code

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HTML code

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