bootstrap php联系表单值显示错误<br /> <b>通知</ b>：未定义变量：<b> C：\ x \ testForm.php </ b>中的myThema <b> 105 </ b> <br />

I am using bootstrap and I want to create a contact Form. I have create a File with the name testForm.php and there I have written my php and my html code(should I make a different files? one for php and one for html?). My file starts with the php code to and after the html code. As soon as I put an <?php echo.... in the html area, everywhere, appears all the time in the site Undefined index: .for example

Now I m trying only with one parameter :thema to see if it works and how it works and if I put something to value comes the result like the picture above. My php code:

<?php
if (isset($_POST["submit"])) {
    $thema = $_POST['thema'];



    // Check if thema has been entered
    if (!$_POST['thema']) {
        $errThema = 'Please enter your thema';
    }


}

?>

and my html code:

<!-- MAIN SEITE -->
<h4><p class="text-primary"><u>Neuanforderungsformular</u></p></h4>

<!-- START FORM -->
<form method="post" role="form" action="testForm.php">
<!-- Thema Feld -->
<div class="form-group">
    <label for="usr">Thema / Title:</label>
    <input type="text" id="thema" name="thema" class="form-control input-lg" value="<?php echo htmlspecialchars($_POST['thema']); ?>">
</div>
<!-- Email Adresse Feld-->
<div class="form-group">
    <label for="exampleInputEmail1">Email address</label>
    <div class="input-group col-sm-5">
    <input type="text" class="form-control input-lg" placeholder="Ihre Email Addresse" aria-describedby="basic-addon2">
    <span class="input-group-addon" id="basic-addon2">@teswt.de</span>
</div>

how can I fix the provbem so my contact form will works?

Try this:

<?php
if (isset($_POST["submit"])) {
    if (isset($_POST['thema'])){
        $thema = $_POST['thema'];
    } else {
        $thema = 'Please enter your thema';
    }
}
?>

You should check first if $thema has been set. What you did is you are using $_POST['thema'] without checking, thus the error appearing in the text field

You are accessing to $_POST['thema']) also when an entry for thema is not present in $_POST

<div class="form-group">
    <label for="usr">Thema / Title:</label>
<input type="text" id="thema" name="thema" class="form-control input-lg" 
        value="<?php echo htmlspecialchars($_POST['thema']); ?>">
 </div>

then you should use a proper setting for a var eg $myThema

if (isset($_POST["submit"])) {
      if (isset($_POST['thema'])){
          $myThema = $_POST['thema'];
      } else {
          $errThema = 'Please enter your thema';
          $myThema = '';
      }
  }

.

<div class="form-group">
    <label for="usr">Thema / Title:</label>
<input type="text" id="thema" name="thema" class="form-control input-lg" 
        value="<?php echo htmlspecialchars($myThema); ?>">
 </div>

I would use something like that:

function GetPostOrDefault($key, $default = '')
{
    if (array_key_exists($key, $_POST))
    {
        return $_POST[$key];
    }
    return $default;
}

And then

<input value="<?= GetPostOrDefault('thema') ?>">

or

<input value="<?= GetPostOrDefault('thema', 'Neues Thema') ?>">

You get this error because the index is not defined, if you have made no post.

That means $_POST['thema'] is only available if you have submitted a form that contains a field with the name thema. On your initial page load, you do a GET request. The form is not submitted.

use like this

$thema = isset($_POST['thema']) ? $_POST['thema'] : '';

instead of

$thema = $_POST['thema'];

update.1

try it

<?php $thema = !empty($thema) ? $thema : ''; ?>
<input type="text" id="thema" name="thema" class="form-control input-lg" value="<?php echo htmlspecialchars($thema); ?>">

instead of

<input type="text" id="thema" name="thema" class="form-control input-lg" value="<?php echo htmlspecialchars($_POST['thema']); ?>">

Simply move your default $thema value outside of the check for $_POST['submit']. Your current code only sets it when the form has been submitted.

$thema = '';
$errThema = '';

if (isset($_POST["submit"])) {
    // Check if thema has been entered
    if (!$_POST['thema']) {
        $errThema = 'Please enter your thema';
    } else {
        $thema = $_POST['thema'];
    }
}

Of course, you should then display $thema instead of $_POST['thema'] in your form.