How can the warning "mysql_num_rows(): supplied argument is not a valid MySQL result resource" be resolved in PHP?

The warning "mysql_num_rows(): supplied argument is not a valid MySQL result resource" indicates that the argument passed to the mysql_num_rows function is not a valid result resource from a MySQL query. This can happen if the query fails or returns an error. To resolve this issue, you should check if the query was successful before using mysql_num_rows to avoid passing an invalid result resource.

// Check if the query was successful before using mysql_num_rows
$result = mysql_query("SELECT * FROM table");
if($result){
    $num_rows = mysql_num_rows($result);
    echo "Number of rows: " . $num_rows;
} else {
    echo "Error executing query: " . mysql_error();
}