What is the potential issue with the error message "Warning: mysql_num_rows(): supplied argument is not a valid MySQL result resource" in PHP?

The potential issue with the error message "Warning: mysql_num_rows(): supplied argument is not a valid MySQL result resource" in PHP is that the function `mysql_num_rows()` expects a valid MySQL result resource as its argument, typically obtained from a query execution. This error indicates that the argument being passed is not a valid result resource, which could be due to a failed query execution or incorrect handling of the result resource. To solve this issue, you should ensure that the query is executed successfully and that the result resource is properly handled before being passed to `mysql_num_rows()`.

// Assuming $conn is the MySQL connection object and $query is the SQL query
$result = mysqli_query($conn, $query);

if($result) {
    $num_rows = mysqli_num_rows($result);
    echo "Number of rows: " . $num_rows;
} else {
    echo "Error executing query: " . mysqli_error($conn);
}