如何验证PHP中的复选框？ [重复]

I have a problem with validating a checkbox. I want to show error message if "submit" is clicked while checkbox is unchecked. With the code I have right now, form gets submitted anyway. I've tried multiple code snippets I found from other questions but none seem to work..

HTML:

<?php include 'contact-form.php'; ?>

<form id="contact" action="<?php $_SERVER['PHP_SELF']; ?>" method="post" 
enctype="multipart/form-data">
<h3>Add issue</h3>
<fieldset>
  <input placeholder="Nimi" type="text" tabindex="1" name="thename"  value="<?= $thename ?>" autofocus>
    <div class="error"><span><?= $name_error ?></span></div>
</fieldset>
<fieldset>
  <input placeholder="Email" type="text" tabindex="2" name="email" value="<?= $email ?>">
    <div class="error"><span><?= $email_error ?></span></div>
</fieldset>
<fieldset>
<input type="checkbox" name="checkbox1" value="yes"/> Annan ühendusele loa minu eluloo avaldamiseks siinsel kodulehel. <br>
  <div class="error"><span><?= $checkbox1_error ?></span></div>
</fieldset>
</form>

PHP:

if ($_SERVER["REQUEST_METHOD"] == "POST") {

if (empty($_POST["thename"])) {
    $name_error = "Palun sisesta nimi";
} else {
    $thename = test_input($_POST["thename"]);
    // check if name only contains letters, whitespace and hyphen
    if (!preg_match("/^[a-zA-Z -]*$/",$thename)) {
        $name_error = "Sisestada saab ainult tähti, tühikuid ja sidekriipse";
    }
}

if (empty($_POST["email"])) {
  $email_error = "Palun sisesta email";
} else {
    $email = test_input($_POST["email"]);
    // email validation
    if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
        $email_error = "Sisesta email korrektselt";
    }
}    

if(!isset($_POST['checkbox1'])) {
  //checkbox is not checked
  $checkbox1_error = 'Väli peab olema kinnitatud';
}

if ($name_error == '' and $email_error == '' and $message_error == '' and $attachment_error == '' and $checkbox1_error == ''){
    $message_body = '';
    unset($_POST['submit']);
    foreach ($_POST as $key => $value){
        $message_body .= "$key: $value
";
    }


    $to = 'myemail@gmail.com';
    $subject = 'My subject';
    $message = "My message";

if (@mail($to, '=?UTF-8?B?' . base64_encode($subject) . '?=', $datamsg, $headers, "-f$email")){
        $success = "Aitäh, kiri edukalt saadetud!";
    } else {
        $error = "Saatmine ebaõnnestus. Palun proovige uuesti.";
    }

Any help is appreciated!

</div>

I used this code and its showing the error message for checkbox

<?php

if ($_SERVER["REQUEST_METHOD"] == "POST") {

    if (empty($_POST["thename"])) {
        $name_error = "Palun sisesta nimi";
    } else {
        $thename = $_POST["thename"];
        // check if name only contains letters, whitespace and hyphen
        if (!preg_match("/^[a-zA-Z -]*$/", $thename)) {
            $name_error = "Sisestada saab ainult tähti, tühikuid ja sidekriipse";
        }
    }

    if (empty($_POST["email"])) {
        $email_error = "Palun sisesta email";
    } else {
        $email = $_POST["email"];
        // email validation
        if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
            $email_error = "Sisesta email korrektselt";
        }
    }

    if (!isset($_POST['checkbox1'])) {
        //checkbox is not checked
        $checkbox1_error = 'Väli peab olema kinnitatud';
    }

    if (!isset($name_error) and ! isset($email_error) and ! isset($message_error) and ! isset($attachment_error) and ! isset($checkbox1_error)) {
        $message_body = '';
        unset($_POST['submit']);
        foreach ($_POST as $key => $value) {
            $message_body .= "$key: $value
";
        }


        $to = 'myemail@gmail.com';
        $subject = 'My subject';
        $message = "My message";

        if (@mail($to, '=?UTF-8?B?' . base64_encode($subject) . '?=', $datamsg, $headers, "-f$email")) {
            $success = "Aitäh, kiri edukalt saadetud!";
        } else {
            $error = "Saatmine ebaõnnestus. Palun proovige uuesti.";
        }
    }
}
?>
<form id="contact" action="<?php $_SERVER['PHP_SELF']; ?>" method="post" 
      enctype="multipart/form-data">
    <h3>Add issue</h3>
    <fieldset>
        <input placeholder="Nimi" type="text" tabindex="1" name="thename"  value="<?php echo $_POST['thename'] ?>" autofocus>
        <div class="error"><span><?php echo $name_error ?></span></div>
    </fieldset>
    <fieldset>
        <input placeholder="Email" type="text" tabindex="2" name="email" value="<?php echo $_POST['email'] ?>">
        <div class="error"><span><?php echo $email_error ?></span></div>
    </fieldset>
    <fieldset>
        <input type="checkbox" name="checkbox1" value="yes"/> Annan ühendusele loa minu eluloo avaldamiseks siinsel kodulehel. <br>
        <div class="error"><span><?php echo $checkbox1_error ?></span></div>
    </fieldset>
    <input type="submit" value="Submit"/>
</form>