HTML文件中的PHP回显

I'm trying to output some text stored in a variable in my HTML file

    <span class="error">*<?php echo $piIdError;?></span>

I have declared and initialized the variable already along with the rest of some other php code that works

            if (empty($_POST['piId']))
    {
        $piIdError = "Pi Id is Required";
    }
    else
    {
        $id = $_POST['piId'];
    }

but when I run the file I get this error:

Notice: Undefined variable: piIdError in C:\xampp\htdocs\piWebConfig\index.php on line 86

Anyone have any ideas to what might be happening?

Thanks

Just initialize the variable $piIdError with the default value like

$piIdError = '';
if (empty($_POST['piId']))
{
    $piIdError = "Pi Id is Required";
}
else
{
    $id = $_POST['piId'];
}

Because if the condition failes then it goes for the else part at where the $piIdError was not defined.Orelse you can use isset like

<span class="error">*
    <?php if(isset($piIdError))
        echo $piIdError;?>
</span>

In your HTML code, use isset() to check if the variable is declared. You can pair it with a ternary operator, and you're all set:

<span class="error"><?php echo (isset($piIdError)) ? $piIdError : ''; ?></span>

<?php
if ($_POST['piId'] == '') {
    $piIdError = "Pi Id is Required";
} else {
    $id = $_POST['piId'];
}
?>

<?php
if(isset($piIdError)) {
    echo '<span class="error">*'.$piIdError.'</span>';
}
?>