Hallo, geht es vielleicht einfacher? Mit weniger Code?

<?php
  $servername = "localhost";
  $username = "root";
  $password = "";

  // Funktion um Verbindung zu prüfen und eine Nachricht auszugeben
  function checkConnection($conn) {
    if ($conn->connect_error) {
      die("Connection failed: " . $conn->connect_error);
    }
  }

  // Verbindung herstellen
  $conn = new mysqli($servername, $username, $password);
  checkConnection($conn);

  // Alle Datenbanken abrufen
  $sql = "SHOW DATABASES";
  $result = $conn->query($sql);
  echo "<h1>Datenbanken</h1>";

  if ($result->num_rows > 0) {
    while ($row = $result->fetch_assoc()) {
      $dbname = $row['Database'];
      echo "<a href='?database=$dbname'>$dbname</a><br>";
    }
  }
  else {
    echo "Keine Datenbanken gefunden.";
  }

  $conn->close();

  // Tabellen in der gewählten Datenbank anzeigen
  if (isset($_GET['database'])) {
    $dbname = $_GET['database'];
    $conn = new mysqli($servername, $username, $password, $dbname);
    checkConnection($conn);
    $sql = "SHOW TABLES";
    $result = $conn->query($sql);
    echo "<h1>Tabellen in $dbname</h1>";

    if ($result->num_rows > 0) {
      while ($row = $result->fetch_assoc()) {
        $tablename = $row["Tables_in_$dbname"];
        echo "<a href='?database=$dbname&table=$tablename'>$tablename</a><br>";
      }
    }
    else {
      echo "Keine Tabellen gefunden.";
    }

    $conn->close();
  }

  // Einträge in der gewählten Tabelle anzeigen und Eingabefelder generieren
  if (isset($_GET['database']) && isset($_GET['table'])) {
    $dbname = $_GET['database'];
    $tablename = $_GET['table'];
    $conn = new mysqli($servername, $username, $password, $dbname);
    checkConnection($conn);
    $sql = "SELECT * FROM $tablename";
    $result = $conn->query($sql);
    echo "<h1>Einträge in $tablename bearbeiten</h1>";

    if ($result->num_rows > 0) {
      echo "<form method='POST' action='save_changes.php'>";
      echo "<table border='1'><tr>";

      // Spaltenüberschriften
      $fieldinfo = $result->fetch_fields();

      foreach ($fieldinfo as $val) {
        echo "<th>{$val->name}</th>";
      }

      echo "</tr>";

      // Daten
      while ($row = $result->fetch_assoc()) {
        echo "<tr>";

        foreach ($row as $key => $data) {
          echo "<td><input type='text' name='{$key}[]' value='$data'></td>";
        }

        echo "</tr>";
      }

      // Leeres Eingabefeld für neue Einträge hinzufügen
      echo "<tr>";

      foreach ($fieldinfo as $val) {
        echo "<td><input type='text' name='{$val->name}[]' value=''></td>";
      }

      echo "</tr>";
      echo "</table>";
      echo "<input type='hidden' name='database' value='$dbname'>";
      echo "<input type='hidden' name='table' value='$tablename'>";
      echo "<input type='submit' value='Änderungen speichern'>";
      echo "</form>";
    }
    else {
      echo "Keine Einträge gefunden.";

      // Leere Tabelle mit Eingabefeldern für neuen Eintrag anzeigen
      echo "<form method='POST' action='save_changes.php'>";
      echo "<table border='1'><tr>";

      foreach ($fieldinfo as $val) {
        echo "<th>{$val->name}</th>";
      }

      echo "</tr><tr>";

      foreach ($fieldinfo as $val) {
        echo "<td><input type='text' name='{$val->name}[]' value=''></td>";
      }

      echo "</tr></table>";
      echo "<input type='hidden' name='database' value='$dbname'>";
      echo "<input type='hidden' name='table' value='$tablename'>";
      echo "<input type='submit' value='Änderungen speichern'>";
      echo "</form>";
    }

    $conn->close();
  }
?>

Mehrere Formularfunktionen in PHP:

Delete:

  if ($_SERVER["REQUEST_METHOD"] == "POST" && isset($_POST["kunden_id"])) {
    $kunden_id = $_POST["kunden_id"];
    $sql = "DELETE FROM kunden WHERE kunden_id=$kunden_id";

    if ($conn->query($sql) === TRUE) {
      echo "Record deleted successfully";
    }
    else {
      echo "Error deleting record: " . $conn->error;
    }
  }

  $conn->close();
?>

Insert:

  if ($_SERVER["REQUEST_METHOD"] == "POST") {
    $vname = $conn->real_escape_string($_POST['vname']);
    $nname = $conn->real_escape_string($_POST['nname']);
    $age = $conn->real_escape_string($_POST['age']);
    $sql = "INSERT INTO kunden (vname, nname, age) VALUES ('$vname', '$nname', '$age')";

    if ($conn->query($sql) === TRUE) {
      echo "Neuer Kunde erfolgreich hinzugefügt";
    }
    else {
      echo "Fehler: " . $sql . "<br>" . $conn->error;
    }
  }

  $conn->close();
?>

Update:

  if ($_SERVER["REQUEST_METHOD"] == "POST" && isset($_POST["kunden_id"]) && isset($_POST["vname"]) && isset($_POST["nname"]) && isset($_POST["age"])) {
    // Kunden-ID und neue Daten aus dem Formular abrufen
    $kunden_id = $_POST["kunden_id"];
    $vname = $_POST["vname"];
    $nname = $_POST["nname"];
    $age = $_POST["age"];

    // SQL-Update-Statement vorbereiten
    $sql = "UPDATE kunden SET vname='$vname', nname='$nname', age='$age' WHERE kunden_id=$kunden_id";

    if ($conn->query($sql) === TRUE) {
      echo "Record updated successfully";
    }
    else {
      echo "Error updating record: " . $conn->error;
    }
  }

  $conn->close();
?>

Search:

  if (isset($_POST['search'])) {
    $search = $_POST['search'];
    $sql = "SELECT kunden_id, vname, nname, age FROM kunden WHERE nname LIKE '%$search%' ORDER BY kunden_id";
  }
  else {
    $sql = "SELECT kunden_id, vname, nname, age FROM kunden ORDER BY kunden_id";
  }

  $result = $conn->query($sql);

  if ($result->num_rows > 0) {
    echo "<table border='1'>
      <tr>
        <th>Kunden ID</th>
        <th>Vorname</th>
        <th>Nachname</th>
        <th>Alter</th>
      </tr>";

    while ($row = $result->fetch_assoc()) {
      echo "<tr>
        <td>" . $row["kunden_id"]. "</td>
        <td>" . $row["vname"]. " </td>
        <td>" . $row["nname"]. "</td>
        <td>" . $row["age"]. "</td>
      </tr>";
    }

    echo "</table>";
  }
  else {
    echo "0 Ergebnisse";
  }

  $conn->close();
?>
<form method="post">
  <input type="text" name="search" placeholder="Nachname eingeben...">
  <button type="submit">Suchen</button>
</form>