php引用用不了...

两个php文件PHPTest.php引用In.php，怎么include那句就不执行了？书上明明这么写的
//PHPTest.php代码
<html>
<head>
<title>PHP</title>
</head><body>
   <?php
       echo"include使用<br>";
       include（"In.php");
   ?>
</body>
</html>//In.php代码<?php
       echo "引用成功！";
?>