PHP不适用于空

我正在构建一个表单，以便用户可以添加自己的产品在网站上销售。javascript应该检查是否填写了所有字段，尽管它不起作用。为什么javascript没有运行？

PHP稍微复杂一些。该网站以不同的包装销售产品，以允许批发和零售定价，因此不同数量的单元有多种定价选项。然而，如果填写了其中一个可选的定价选项或单位编号，但没有填写相应的值，我认为这会导致后续问题，除了用户打字错误之外，没有其他原因导致这种情况发生。

为了避免这种情况，如果设置了price2，而units2为空，我希望表单被拒绝，并回复"您需要为每个套餐选项填写单位和价格"。倒数、price3和units3也是如此。

这就是PHP应该做的。然而，第一个"if"每次都会触发，所以"你需要为每个包填写单位和价格option1"会出现回声。为什么运行不正确？

<script>
function validproform()
{
var a=document.forms["productentry"]["pname"].value;
var b=document.forms["productentry"]["pword"].value;
var c=document.forms["productentry"]["about"].value;
var d=document.forms["productentry"]["unitabout"].value;
var e=document.forms["productentry"]["price1"].value;
var f=document.forms["productentry"]["units1"].value;
if (a==null || a=="" || b==null || b=="" || c==null || c=="" || d==null || d=="" || e==null || e=="" || f==null || f=="")
  {
  alert("Required fields are not all filled out.");
  return false;
  }
</script>
<?php
if (isset($_POST['psubmit'])) {
    if(isset($_POST['price2']) and empty($_POST['units2'])){
        echo('You need to fill out both units and prices for each package option1');
    }
    elseif(isset($_POST['units2']) and empty($_POST['price2'])){
        echo('You need to fill out both units and prices for each package option2');
    }
    elseif(isset($_POST['price3']) and empty($_POST['units3'])){
        echo('You need to fill out both units and prices for each package option3');
    }
    elseif(isset($_POST['units3']) and empty($_POST['price3'])) {
        echo('You need to fill out both units and prices for each package option4');
    }
}
?>
<form name="productentry" action="" method="post" onsubmit="return validproform()">
Product Name: <input type="text" name="pname" maxlength="30"><br>
About: <input type="text" name="about" maxlength="250"><br>
Describe your product. What is unique about it, what does it do, what sets it apart?<br>
What each unit contains: <input type="text" name="unitabout" maxlength="250"><br>
For example: Are you selling, "A bag of chips" or "A box w/ 30 bags of chips" or "a carton of 10 boxes with 30 bags of chips each." Describe what one unit contains.<br>
Price: <input type="text" name="price1" maxlength="30"><br>
Number of units: <input type="text" name="units1" maxlength="30"><br>
----
Price 2(optional): <input type="text" name="price2" maxlength="30"><br>
Number of units(optional): <input type="text" name="units2" maxlength="30"><br>
Price 3(optional): <input type="text" name="price3" maxlength="30"><br>
Number of units(optional): <input type="text" name="units3" maxlength="30"><br>
Password: <input type="password" name="pword" maxlength="30"><br>
<input type="submit" name="psubmit">
</form>