<body>


  <form method="post" action="">
 placeholder
  <input name="email" type="text" placeholder="这里输入文字" size="22" />

  <br>
javascript
  <input name="mail" type="text" value="请输入内容" onfocus="javascript:if(this.value=='请输入内容')this.value='';">    

  <input type="submit">
  </form>

<hr>
<?php
var_dump($_POST);
?>
 </body>