Erwartet genau zwei Parameter:

mysqli_select_db($host,$user,$passwort,"fcnienburg49"); geht nicht!

"Warning: mysqli_select_db() expects exactly 2 parameters, 4 given in /users/fcnienburg49/www/Latein-Eintrag2.php on line 33"

Darum: mysqli_select_db($link,"$dbname"); oder
mysqli_select_db($link,$dbname);

<?php
  $con=mysqli_connect("localhost","my_user","my_password","my_db");
  // Check connection
  if (mysqli_connect_errno())
    {
    echo "Failed to connect to MySQL: " . mysqli_connect_error();
    }

// ...some PHP code for database "my_db"...

// Change database to "test"
mysqli_select_db($con,"test");

// ...some PHP code for database "test"...

mysqli_close($con);
?>