表单提交后保留输入值（带有catch）

In PHP, if the text input "cmtx_comment" is empty, on form submit I show a javascript alert. After I press OK in the alert, the values entered by the user in all fields in the form are gone. How can I keep the user entered values, without adding code to the value of the input elements (something like <input type="text" name="something" value="<?php echo $_GET['something'];?>"> ?

if (empty($cmtx_comment)) { //if comment value is empty
echo <<<EOD
<script>
alert('Please enter a comment!');
</script>
EOD;
return false;
} else { //if comment entered
do stuff

Have you tried localStorage and form validation?

HTML:

<form method="post" action="" onSubmit="return saveComment();">
    <input type="text" name="cmtx_comment" id="cmtx_comment" value="" />
    <input type="submit" value="Save" />
</form>

JavaScript:

document.getElementById("cmtx_comment").value = localStorage.getItem("comment");

function saveComment() {
    var comment = document.getElementById("cmtx_comment").value;
    if (comment == "") {
        alert("Please enter a comment in first!");
        return false;
    }

    localStorage.setItem("comment", comment);
    alert("Your comment has been saved!");

    location.reload();
    return false;
    //return true;
}

Example

On first page load, you are presented with:

If you don't enter a comment, you get the alert:

If you do enter a comment, you get a different alert:

The page will then refresh (or post, simply un-comment the return true, and comment the location.reload), and you will still see the contents you posted the first time.