PHP Form Error Message on Form Page

I am pretty new to PHP and I can't seem to find what I am looking for. I want the error to show on the page where the form is filled out. I want a "Please enter name" error to show if it's blank in the $showerror part of the form page.

This is what I have so far...

<form method="post" action="process.php">
<table width="667">
<td colspan="2"><?php echo $showerror; ?></td>
<tr>
<td>Full Name:*</td>
<td><input class="text" name="name" placeholder="Ex. John Smith"></td>
<tr>
<td>E-mail:*</td>
<td><input class="text" name="email" placeholder="Ex. [email protected]"></td>
<tr>
<td>Type of site:*</td>
<td>Business<input name="multioption[]" type="radio" value="Business" class="check" /> Portfolio<input name="multioption[]" type="radio" value="Portfolio" class="check" /> Forum<input name="multioption[]" type="radio" value="Forum" class="check" /> Blog<input name="multioption[]" type="radio" value="Blog" class="check" /></td>
<tr>
<td>Anti-Spam: (2)+2=?*</td>
<td><input name="human" placeholder="What is it?"></td>
</table>
<input id="submit" name="submit" type="submit" value="Submit"></form>

Then this is what my process.php looks like... I am not sure how to code the error part.

<?php
$name         = isset($_POST['name'])         ? $_POST['name']         : '';
$email        = isset($_POST['email'])        ? $_POST['email']        : '';
$human        = isset($_POST['human'])        ? $_POST['human']        : '';
$submit       = isset($_POST['submit'])       ? true                   : false;

$multioption = isset($_POST['multioption'])
         ? implode(', ', $_POST['multioption'])
         : 'No multioption option selected.';



$from = 'From: Testing Form'; 
$to = '[email protected]'; 
$subject = 'Testing Form';

$body = 
"Name: $name\n 
E-Mail: $email\n 
Multi Options: $multioption\n";

if ($submit && $human == '4') {
    mail ($to, $subject, $body, $from);
    print ("Thank you. We have received your inquiry.");

}
else {
   echo "We have detected you are a robot!.";
    }
?>

Solution

You need to place PHP syntax between PHP tags, for example, this line:

<td colspan="2">$showerror</td>

becomes like this:

<td colspan="2"><?php echo $showerror; ?></td>

If you're completely new to PHP, you can start learning from some tutorial websites, here's a good one

EDIT:

You can set $showerror in the PHP page, this could be a long list of "if conditions" for each form field, but I will show you a small/simple example for the full-name $_POST['name'], it will be like this:

$showerror = '';
if(!empty($_POST['name'])) {// enter here if fullname is set
    if(strlen($_POST['name']) >= 6 && strlen($_POST['name']) <= 12) {// enter here if fullname length is between 6-12 characters
        // You can do more validation by using "if conditions" here if you would like, or keep it empty if you think fullname is correct
    } else {// enter here if fullname is NOT between 6-12 characters
        $showerror = 'Full name must be 6-12 characters';
    }
} else {// enter here if fullname is not set
    $showerror = 'Please enter your full name';
}