whats wrong with this php mysql_real_escape_string

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* check the variables for content */

/*** a list of filters ***/
$filters = array(
'searchtext' => array( 'filter' => FILTER_CALLBACK, 'options'  => 'mysql_real_escape_string'),
'fieldname'  => array( 'filter' => FILTER_CALLBACK, 'options'  => 'mysql_real_escape_string')
);

/*** escape all POST variables ***/
$input = filter_input_array(INPUT_POST, $filters);

/*** check the values are not empty ***/
if(empty($input['fieldname']) || empty($input['searchtext']))
    {
    echo 'Invalid search';
    }
else
    {
    /*** mysql hostname ***/
    $hostname = 'localhost';

    /*** mysql username ***/
    $username = 'username';

    /*** mysql password ***/
    $password = 'password';

    /*** mysql database name ***/
    $dbname = 'periodic_table';

    /*** connect to the database ***/
    $link = @mysql_connect($hostname, $username, $password);

    /*** check if the link is a valid resource ***/
    if(is_resource($link))
        {
        /*** select the database we wish to use ***/
        if(mysql_select_db($dbname, $link) === TRUE)
            {
            /*** sql to SELECT information***/
        $sql = sprintf("SELECT * FROM elements WHERE %s = '%s'", $input['fieldname'], $input['searchtext']);

        /*** echo the sql query ***/
        echo '<h3>'.$sql.'</h3>';

            /*** run the query ***/
            $result = mysql_query($sql);

            /*** check if the result is a valid resource ***/
            if(is_resource($result))
                {
                /*** check if we have more than zero rows ***/
                if(mysql_num_rows($result) !== 0)
                    {
            echo '<table>';
                    while($row=mysql_fetch_array($result))
                        {
                        echo '<tr>
                        <td>'.$row['atomicnumber'].'</td>
                        <td>'.$row['latin'].'</td>
                        <td>'.$row['english'].'</td>
                        <td>'.$row['abbr'].'</td>
                        </tr>';
                        }
            echo '</table>';
                    }
                else
                    {
                    /*** if zero results开发者_如何学C are found.. ***/
                    echo 'Zero results found';
                    }
                }
            else
                {
                /*** if the resource is not valid ***/
                'No valid resource found';
                }
            }
        /*** if we are unable to select the database show an error ****/
        else
            {
            echo 'Unable to select database '.$dbname;
            }
        /*** close the connection ***/
        mysql_close($link);
        }
    else
        {
        /*** if we fail to connect ***/
        echo 'Unable to connect';
        }
    }
}

else { echo 'Please Choose An Element'; } ?>

I got this code from phppro.org tutorials site and i tried to run it. It gives Warning: mysql_real_escape_string() [function.mysql-real-escape-string]: A link to the server could not be established. .... Warning: mysql_real_escape_string() [function.mysql-real-escape-string]: Access denied for user 'ODBC'@'localhost' (using password: NO)....

I went to php.net and look it up "Note: A MySQL connection is required before using mysql_real_escape_string() otherwise an error of level E_WARNING is generated, and FALSE is returned. If link_identifier isn't defined, the last MySQL connection is used."

My questions are: 1-why they put single quotation around mysql_real_escape_string ? 2-They should establish a connection first, then use the $filter array statement with mysql_real_escape_string ?

Your problem here is not with mysql_real_escape_string. Your problem is with the fact that you don't have a connection to a server. Do you have a database 'periodic_table' running locally with 'username' and 'password' for your login credentials?

1. they used function name as a parameter for the another function.
2. Yes

I think mysql_real_escape_string needs a connection so it knows what charset is used.

also in php, single quotes are faster then double quotes as php doesn't have to process the contents of thr string looking for $varname etc.