What could be causing the error message "Warning: mysql_num_rows(): supplied argument is not a valid MySQL result resource" in PHP?

The error message "Warning: mysql_num_rows(): supplied argument is not a valid MySQL result resource" typically occurs when the query executed in PHP does not return a valid result set. This can happen due to issues with the SQL query syntax, database connection problems, or incorrect usage of the mysql functions. To solve this issue, you should check your SQL query for errors, verify the database connection, and ensure that the result resource is valid before using it in mysql_num_rows().

// Assuming $conn is the database connection variable and $query is the SQL query
$result = mysql_query($query, $conn);

if($result){
    $num_rows = mysql_num_rows($result);
    // Continue processing the result set
} else {
    echo "Error executing query: " . mysql_error();
}