PHP SQL While loop not working

You're killing your query by reconnecting to the DB inside your loop

$con = mysqli_connect(...) // connection #1
$result = mysqli_query(...);
while($row = mysqli_fetch($result)) {
   $con = mysqli_connect(...); // connection #2

When you connect again, you kill the original connection, which kills your query.

Unless you need to connect twice with different credentials, there is NO need for a second connection. One single connection can handle multiple queries.

Incidentally, if you'd used a different connection handle variable, e.g.

$con = mysqli_connect(...);
    $othercon = mysqli_connect(...);

you wouldn't have had the problem. You CAN have multiple connections, but not using the same single variable.

You have two MySQL connections with the same variable names of $con as well as $result & $row. So I just change the variable names on the inside loop so they don’t conflict & all should work; $con_inside, $result_inside & $row_inside.

I also added or die(mysqli_error()); to your mysqli_query lines so errors can be returned if your query dies.

<?php
$con = mysqli_connect("localhost","UN","PW","DB");

// Check connection
if (mysqli_connect_errno()) {
  echo "Failed to connect to MySQL: " . mysqli_connect_error();
}

$id = $_SESSION['user']['id'];

$result = mysqli_query($con, "SELECT * FROM favourites WHERE user='$id'") or die(mysqli_error());

while ($row = mysqli_fetch_array($result)) {

  $code = $row['gamecode'];

  $con_inside = mysqli_connect("localhost","UN","PW","DB");

  // Check connection
  if (mysqli_connect_errno()) {
    echo "Failed to connect to MySQL: " . mysqli_connect_error();
  }

  $result_inside = mysqli_query($con_inside, "SELECT * FROM menus WHERE code='$code'") or die(mysqli_error());

  while($row_inside = mysqli_fetch_array($result_inside)) {
?>

  <a href="<?php echo $row_inside['link']; ?>">
  <img src="<?php echo $row_inside['picture']; ?>" alt="<?php echo $row_inside['game']; ?>" height="120" width="150" class="fade"></a>

<?php
  }

  mysqli_close($con_inside);

}

mysqli_close($con);

Also, here is a slightly reworked version of your code that should work better. I removed the inside DB connection from the loop & set it at the top of the script. The connection does not have to be reset on each loop. Also, I added lines using mysqli_stmt_bind_param which is a preferred way of using mysqli_* queries instead of setting strings. Also using mysqli_free_result to free up query memory on each loop. These are small things but they add up to better code.

<?php

// Main DB connection.
$con = mysqli_connect("localhost","UN","PW","DB") or die(mysqli_connect_error());

// Inside DB connection.
$con_inside = mysqli_connect("localhost","UN","PW","DB") or die(mysqli_connect_error());

// Set the $id variable.
$id = $_SESSION['user']['id'];

// Set the query string.
$query = "SELECT * FROM favourites WHERE user='$id'";

// Bind the values to the query.
mysqli_stmt_bind_param($query, 's', $id);

// Get the result.
$result = mysqli_query($con, $query) or die(mysqli_error());

// Roll through the results.
while ($row = mysqli_fetch_array($result)) {

  // Set the $code variable.
  $code = $row['gamecode'];

  // Set the query string.
  $query_inside = "SELECT * FROM menus WHERE code='$code'";

  // Bind the values to the query.
  mysqli_stmt_bind_param($query_inside, 's', $code);

  // Get the result.
  $result_inside = mysqli_query($con_inside, $query_inside) or die(mysqli_error());

  // Roll through the results.
  while($row_inside = mysqli_fetch_array($result_inside)) {
?>

  <a href="<?php echo $row_inside['link']; ?>">
  <img src="<?php echo $row_inside['picture']; ?>" alt="<?php echo $row_inside['game']; ?>" height="120" width="150" class="fade"></a>

<?php
  }

  // Free the result set.
  mysqli_free_result($result_inside);

  // Close the connection.
  mysqli_close($con_inside);

}

// Free the result set.
mysqli_free_result($result);

// Close the connection.
mysqli_close($con);

See if it works using different result variables and only one DB connection.

<?php
$con=mysqli_connect("localhost","UN","PW","DB");
// Check connection
if (mysqli_connect_errno()) {  echo "Failed to connect to MySQL: " . mysqli_connect_error(); }
$id = $_SESSION['user']['id'];
$result1 = mysqli_query($con,"SELECT * FROM favourites WHERE user='$id'");
while($row = mysqli_fetch_array($result1)) {
    $result2 = mysqli_query($con,"SELECT * FROM menus WHERE code='".$row['gamecode']."");
    while($row2 = mysqli_fetch_array($result2)) { ?>
<a href="<?php echo $row2['link']; ?>">
<img src="<?php echo $row2['picture']; ?>" alt="<?php echo $row2['game']; ?>"      height="120" width="150" class="fade"></a>
<?php }
}
mysqli_close($con);
?>