How to check if username already registered

mysqli_num_rows() can only be used with SELECT queries, it returns the number of rows in the result set.

To test how many rows were affected by a modification query, use mysqli_affected_rows(). And the argument to this should be $connect, not $result (which is just a boolean).

$result = mysqli_query($connect, $query);
if (!$result) {
    die (mysqli_error($connect));
} elseif (mysqli_affected_rows($connect) == 0) {
    exit ("Username already exists");
} else {
    echo '<h3><font color="red">You have successfully registered</font></h3>';

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