//------------------------- HTML:

<div id="error" class="<?php 
if( $error !== '' ){
	echo 'showError';
} ?>">Sie haben nicht alle Felder ausgefüllt</div>
<form name="skillflow" method="post" 
action="form_2.php" enctype="multipart/form-data">
	<span class="label">Anrede: </span>
	<select name="anrede" class="mySelect">
		<option value="frau" <?php 
		if( $error !== '' && $_POST['anrede'] == 'frau' ){ 
			echo 'selected'; 
		} 
		?>>Frau</option>
		<option value="herr" <?php 
		if( $error !== '' && $_POST['anrede'] == 'herr' ){ 
			echo 'selected'; 
		} 
		?>>Herr</option>
		<option value="divers" <?php 
		if( $error !== '' && $_POST['anrede']  == 'divers' ){ 
			echo 'selected'; 
		} 
		?>>Divers</option>
	</select>
	<br>
	<span class="label<?php 
	if($error == 'vorname'){ 
		echo ' label-error';
	} 
	?>">Vorname: </span>
	<input type="text" class="" name="vorname" value="<?php 
	if( isset( $_POST['send']) ){ 
		echo $_POST['vorname']; } 
	?>">
	<br>
	<span class="label<?php 
	if($error == 'name'){ 
		echo ' label-error';
	} 
	?>">Name: </span>
	<input type="text" class="" name="name" value="<?php 
	if( isset( $_POST['send']) ){ 
		echo $_POST['name']; 
	} 
	?>">
	<br>
	<span class="label<?php 
	if($error == 'email'){ 
		echo ' label-error';
	} ?>">E-Mail: </span>
	<input type="email" class="" name="email" value="<?php 
	if( isset( $_POST['send']) ){ 
		echo $_POST['email']; 
	} 
	?>">
	<br>
	<div class="nachricht-text<?php 
	if($error == 'kommentar'){ 
		echo ' label-error';
	} 
	?>">Nachricht:</div>
	<textarea name="kommentar" class="comment">
	<?php 
	if( isset( $_POST['send']) ){ 
		echo $_POST['kommentar']; 
	} 
	?></textarea>
	<br>
	<div class="vocal-text">
		Bitte entferne aus dem Feld unten alle Vokale.
	</div>
	<input type="text" name="vocal" class="vocal-input" value="">
	<br>
	<input type="submit" class="mybutton" name="send" value="Send">
	  
	<input type="reset" class="mybutton" value="Delete">
</form>



//------------------------- CSS:

#error{
	width:100%;
	color:red;
	font-size:14px;
	font-weight:900;
	text-align:center;
	border:1px solid white;
	padding:5px 0;
	margin-bottom:15px;
	visibility:hidden;
}

.label{
	display:inline-block;
	width:75px;
	font-size:14px;
	padding:5px 3px;
	margin-right:5px;
	border:1px solid white;
}

select.mySelect{
	width:auto;
	color:black;
	padding: 1px 0;
	background-color: white;
	border:1px solid white;
}
option{
	color:black;
	background-color: white;
	padding:5px 0;
}

input[type="text"], 
input[type="email"]{
	width:200px;
	font-size:14px;
	padding:5px 0;
	color:black;
	background-color: white;
	border:1px solid white;
}

.nachricht-text{
	display:block;
	font-size:14px;
	margin:15px 0px 5px 0px;
}

textarea.comment{
	width: 95%;
    height: 150px;
	color:black;
	background-color: white;
	border:1px solid white;
}

.vocal-text{
	margin:10px 0px 0px 0px;
	
}

input.vocal-input{
	width: 99%;
    font-size: 18px;
    padding: 3px;
}

.mybutton {
    width: auto;
    padding: 2px 2px;
    color: black;
    background-color: white;
    cursor: pointer;
	border:1px solid white;
}


#error.showError{
	visibility:visible;
}

.label-error{
	font-weight:600;
	color:red !important;
}



//------------------------- PHP:

<?php 
$error = '';
// Erst wenn der Button namens send geklickt wurde
if ( isset( $_POST['send'] ) ) {
	// Die Methode isset() 
	// ermittelt ob $_POST true ist	
	// Um alle Felder zu prüfen erstellen wir ein Array mit den Feldnamen
	$fields = [];
	$fields[0] = 'anrede'; 
	$fields[1] = 'vorname'; 
	$fields[2] = 'name'; 
	$fields[3] = 'email'; 
	$fields[4] = 'kommentar'; 
	
	// foreach-Schleife um die Felder zu prüfen
	foreach ( $fields as $field ) {        
		// Die Methode trim() entfernt Leerzeichen 
		// Links und Rechts von der Eingabe, falls vorhanden
		$value = trim( $_POST[ $field ] );
		if ( $value === '' ) {
			$error = $field;
			break;
		}
	}	
	if ( $error === '' ) {
		// Weiter zum Mailversand
	}
}

?>