What common mistakes can lead to the error message "mysql_num_rows() expects parameter 1 boolean given" in PHP?

The error message "mysql_num_rows() expects parameter 1 boolean given" occurs when the function expects a result set as the parameter, but receives a boolean value instead. This can happen when the query fails to execute properly, returning a boolean false instead of a result set. To solve this issue, you should check if the query was successful before calling mysql_num_rows().

$result = mysql_query($query);

if($result){
    $row_count = mysql_num_rows($result);
    // Process the result set
} else {
    // Handle the query failure
}