Can't use $_post value as table name for mysql in php

Question

I'm working on a blog right now that aims to display messages to only people it belongs to, so I have a select in html where people can select a person and then it sends it to that table in MySQL.

What I now have in the index.html:

<form action="post.php" method="post">
                <label>Naam:</label>
                <input type="text" name="name" placeholder="Naam" class="form-control">
                <label>Voor wie is dit bericht bestemd?</label>
                <select name="portal" class="form-control">
                    <option id="0">Selecteer</option>
                    <option id="1">Leerlingen</option>
                    <option id="2">Docenten</option>
                    <option id="3">Ouders</option>
                    <option id="4">Bedrijven</option>
                </select>
        </div>
        <div class="paper col-sm-6">
                <label>Email:</label>
                <input type="email" placeholder="Email" class="form-control">
                <label>Onderwerp:</label>
                <input type="textarea" class="form-control" placeholder="Onderwerp" name="subject"/>
        </div>
        <div class="paper col-sm-12">
                <label>Korte informatie:</label>
                <input class="form-control" type="textarea" name="short"/>
                <label>Volledige informatie</label>
                <textarea class="form-control" rows="4" cols="50" name="long"></textarea>
        </div>
        <div class="paper col-sm-12 text-center">
                <div class="col-xs-12" style="height:25px;"></div>
                <button class="btn btn-default">Verstuur!</button>
            </form>

and this in my post.php:

<?php
    $servername = "localhost";
    $username = "a1070rik";
    $password = "";
    $dbname = "portals";
    $title = '$_POST[subject]';
    $by = '$_POST[name]';
    $short = '$_POST[short]';
    $long = '$_POST[long]';
    $portal = '$_POST[portal]';

// Create connection
$conn = new mysqli($servername, $username, $password, $dbname);
// Check connection
if ($conn->connect_error) {
    die("Connection failed: " . $conn->connect_error);
} 

$sql = "INSERT INTO $portal (id, title, by, short, long)
VALUES ('', $title, $by, $short, $long)";

if ($conn->query($sql) === TRUE) {
    echo "New record created successfully";
} else {
    echo "Error: " . $sql . "<br>" . $conn->error;
}

$conn->close();
?>

When I try to run it it gives me this vague error:

Error: INSERT INTO $_POST[portal] (id, title, by, short, long) VALUES ('', $_POST[subject], $_POST[name], $_POST[short], $_POST[long])
You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '[portal] (id, title, by, short, long) VALUES ('', $_POST[subject], $_POST[name],' at line 1

Thanks

EDIT:

Thanks everyone that helped,

this code eventually worked for me:

<?php
    $servername = "localhost";
    $username = "a1070rik";
    $password = "";
    $dbname = "portals";
    $title = $_POST['subject'];
    $by_information = $_POST['name'];
    $short = $_POST['short'];
    $long_information = $_POST['long'];
    $portal = $_POST['portal'];

// Create connection
$conn = new mysqli($servername, $username, $password, $dbname);
// Check connection
if ($conn->connect_error) {
    die("Connection failed: " . $conn->connect_error);
} 

$sql = "INSERT INTO  $portal (`id`, `title`, `info_bys`, `info_shorts`, `info_longs`) VALUES ('', '$title', '$by', '$short', '$long')";

if ($conn->query($sql) === TRUE) {
    echo "New record created successfully";
} else {
    echo "Error: " . $sql . "<br>" . $conn->error;
}

$conn->close();
?>

Answer 1

$_POST works like an array So you will need to get values from $_POST by his indexs i.e in your case subject,name etc. So Remove ' while assigning the values to variable.

$title = $_POST['subject'];
$by = $_POST['name'];
$short = $_POST['short'];
$long = $_POST['long'];
$portal = strtolower($_POST['portal']);

NOTE : The names 'by,long' are MySQL reserved keywords. So Change them.

Update your SQL from

 $sql = "INSERT INTO $portal (id, title, by, short, long) VALUES ('', '$title', '$by', '$short', '$long')";

TO

 $sql = "INSERT INTO  $portal (`id`, `title`, `info_bys`, `info_shorts`, `info_longs`) VALUES ('', '$title', '$by', '$short', '$long')";

Your sql is vulnerable So use

// prepare and bind
$stmt = $conn->prepare("INSERT INTO  $portal (`title`, `info_bys`, `info_shorts`, `info_longs`) VALUES (?, ?, ?, ?)");
$stmt->bind_param($title, $by, $short, $long);
$stmt->execute();

Answer 2

Dont use variables as strings. Keep it organized and fool proof:

<?php
    $servername = "localhost";
    $username = "a1070rik";
    $password = "";
    $dbname = "portals";
    $title = $_POST['subject'];
    $by = $_POST['name'];
    $short = $_POST['short'];
    $long = $_POST['long'];
    $portal = $_POST['portal'];

// Create connection
$conn = new mysqli($servername, $username, $password, $dbname);
// Check connection
if ($conn->connect_error) {
    die("Connection failed: " . $conn->connect_error);
} 

$portal = $mysqli->real_escape_string($portal);
$title = $mysqli->real_escape_string($title);
$by = $mysqli->real_escape_string($by);
$short = $mysqli->real_escape_string($short);
$long = $mysqli->real_escape_string($long);

$sql = "INSERT INTO `".$portal."` (id, title, by, short, long) VALUES ('', '".$title."', '".$by."', '".$short."', '".$long."')";

if ($conn->query($sql) === TRUE) {
    echo "New record created successfully";
} else {
    echo "Error: " . $sql . "<br>" . $conn->error;
}

$conn->close();
?>