php设置文本框不能为空

<form method="post" action="/check.php">
<input type="text" name="content" id="content" />
<input type="submit" value="提交" />
</form>

<form method="post" action="/check.php">
<!-- 表单改成下面这样 (加了一个 onsubmit) -->
<form method="post" action="/check.php" onsubmit="return checkForm()">

<!-- 然后写一个简单的js判断一下 -->
<script type="text/javascript">
	function checkForm(){
		var tag = false;
		var checkText = document.getElementById("content").value;
		if ( checkText == "" || checkText == null ){
			alert("未输入");
		}else{
			alert("已输入");
			tag = true;
		}
		return tag;
	}
</script>

<script type="text/javascript" src="/checkform.js">

<?php
$checkText = $_POST['content'];
if ( empty( $checkText ) ){
    echo '<script type="text/javascript">alert("未填写");historty.go(-1)</script>';
    exit; //写exit是很必要的, 防止浏览器禁用JS.
}
//这里不用else了,能执行到这一行不用再判断
echo '<script type="text/javascript">alert("未填写");historty.go(-1)</script>';
?>