Store data from array to database use checkbox

I want to store some data/list from array with checkboxes to the database, so I checked the list that will entrance to the database:

class function thus:

<?php
class st_exchange_conv {
	var $from, $to, $bid, $ask, $stocks, $localised_stocks, $source ;
	function st_exchange_conv( $source = 'Site_A' ) {
		$this->from = NULL ;
		$this->to = NULL ;
		$this->price = NULL ;
        $this->source = $source ;
        switch ( $source ) {
            case 'Site_A':
                $this->stocks = $this->localised_stocks = array(
				'TSM' => 'Tams Star Management',
				'BVM' => 'Benthul Valid M',
				'GRT' => 'Guard Rising Tld',
				'RPI' => 'RP Invesments',
				'BGS' => 'Belgrado SSd') ;
                break ;
            case 'Site_B':
            default:
                $this->stocks = $this->localised_stocks = array(
				'BCA' => 'Bursa Asi',
				'DZD' => 'Data ZeendetDung',
				'XAL' => 'Aluminium Mining',
				'RPI' => 'RP Invesments',
				'AWG' => 'Aqua Water Gas',
				'VAL' => 'Volun All Lintas',
				'BSD' => 'Bumi SeeDs') ;
                break ;
        }
	}
	function convert( $from, $to, $date = NULL ) {
		$this->from = $from ;
		$this->to = $to ; 
        if ( !$date )
        {  // Date parameter absent - default to now
            $date = time() ; 
        } else if ( !is_numeric( $date ) )
        { // Date parameter isn't a timestamp, so treat as text representation of a date
            $date = strtotime( $date ) ;
        } 
        if ( $from == $to )     {
            $this->price = 1 ;
            return TRUE ;
        }
        switch ( $this->source )
        {
            case 'Site_A':
                $url = 'http://www.site_a.com/conv?a=1&from=' . $from . '&to=' . $to ;
                $pattern = '#\sid=stocks[^0-9]*[0-9.]+[^A-Z]*' . $from . '[^A-Z][^0-9]+([0-9.]+)[^A-Z]*' . $to . '[^A-Z]#' ;
                return $this->fetchSourceData( $url, $pattern, 1, 0, 1, 'curl' ) ;    
                break; 
            case 'Site_B':
            default:
                $url = 'http://site_b.com/conv?e=sv&f=st1&s='. $from . $to .'=V';
                $pattern = '#"' . $from . $to . '=V",([0-9.]*),"#' ;
                $max_matches = 1 ;
                $result_match = 0 ;
                $conv_amt = 1 ;
                $method = 'fopen' ;
                return $this->fetchSourceData( $url, $pattern, $max_matches, $result_match, $conv_amt, $method ) ;           
                break ;
        }
	}
    function fetchSourceData( $url, $pattern, $max_matches, $result_match, $conv_amt, $method="curl" ) {
        // Fetch source page
        switch ( $method ) {
            case 'curl':       
                $ch = curl_init();
                curl_setopt( $ch, CURLOPT_URL, $url ) ;
                curl_setopt( $ch, CURLOPT_RETURNTRANSFER, 1 );
                curl_setopt( $ch, CURLOPT_HEADER, 1 );
                curl_setopt( $ch, CURLOPT_VERBOSE, 1 ) ;
                $page = curl_exec( $ch ) ;
                curl_close ($ch);
                break;
            case 'fopen':
                $ch = @fopen( $url, 'r' ) ;      
                if ( $ch ) {  
                    $page = fgets( $ch, 4096 ) ;  
                    fclose( $ch ) ;  
                } else {
                    $page = 'FAIL' ;
                } 
                break ;
        }
        // Capture result field
        preg_match_all( $pattern, $page, $matches ) ;
        // Lookup has failed unless at least and no more than the maximum expected one non-zero matches were achieved
        if ( count( $matches[1] ) == 0 OR count( $matches[1] ) > $max_matches OR $matches[1][ $result_match ] == 0 ) 
        {			return FALSE;
        } else {
            $decimal_result = $matches[1][ $result_match ] / $conv_amt ;
            if ( !settype( $decimal_result, 'float' ) ) return FALSE ;
            $this->price = $decimal_result ;
            return TRUE ;
        }
    }
	function price( $amount = 1.00, $unused = '' ) {
		return $amount * $this->price ;
	}
	function name( $symbol ) {
		return ( $symbol == 'from' ) ?
			$this->localised_stocks[ $this->from ] : $this->localised_stocks[ $this->to ] ;
	}
	function stocks() {
		return $this->localised_stocks ;
	}
}
?>

PHP:

script stores the database as follows:

session_start();
if(isset($_POST['submit']))
{
	include('class/stockconvert_class.php');
	$st = new st_exchange_conv(DEFAULT_SOURCE);
	$from = mysql_real_escape_string(stripslashes($_POST['from']));
	$value = floatval($_POST['amount']);
	$date = date('Y-m-d H:i:s');
	$_SESSION['selected'] = $from;
	$stocks = $st->stocks();
	asort($stocks);
	foreach($stocks as $key=>$stock)
	{
		$st->convert($from,$key,$date);
		$stc_price = $st->price($value);
		$stock = mysql_real_escape_string(stripslashes($stock));
		$count = "SELECT * FROM oc_stock WHERE stock = '$key'";
		$result = mysql_query($count) or die(mysql_error());
		$sql = '';
		if(mysql_num_rows($result) == 1)
		{
			$sql = "UPDATE oc_stock SET stock_title = '$stock', stc_val = '$stc_price', date_updated = '$date' WHERE stock = '$key'";
		}
		else
		{		
		$sql = "INSERT INTO oc_stock(stock_id,stock_title,stock,decimal_place,stc_val,date_updated) VALUES ('','$stock','$key','2',$stc_price,'$date')";
		}	
		$result = mysql_query($sql) or die(mysql_error().'<br />'.$sql);
	}
	header("Location: index.php");
	exit();
}

PHP:

while from its checkbox as follows:

session_start();
define('DEFAULT_SOURCE','Site_A'); 
define('DEFAULT_VALUE',100);
define('DEFAULT_STC','BGS');
include('class/stockconvert_class.php');
$st = new st_exchange_conv(DEFAULT_SOURCE);
?>
<form action="do.php" method="post">
<label for="amount">Amount:</label>
<input type="input" name="amount" id="amount" value="1"><input type="submit" name="submit" value="Convert">
<?php
$stocks = $st->stocks();
asort($stocks);
foreach($stocks as $key=>$stock)
{
	if((isset($_SESSION['selected']) && strcmp($_SESSION['selected'],$key) == 0) || (!isset($_SESSION['selected']) && strcmp(DEFAULT_STC,$key) == 0))
	{
	?>
	<br><input type="checkbox" id="scb1" name="from[]" value="<?php echo $key; ?>" checked="checked"><?php echo $stock; ?>
	<?php
	}
	else
	{
	?>
	<br><input type="checkbox" id="scb1" name="from[]" value="<?php echo $key; ?>"><?php echo $stock; ?>
	<?php
	}
}
?>

PHP:

and error as follows:
Fatal error: Call to undefined method st_exchange_conv::convert() in C:\xampp\htdocs\test\do.php on line 21

line 21 is $st->convert($from,$key,$date);

there seems to be wrong in its checkbox form, but I expect though more definite clues, Thanks in advance..

 

see this
see the app in the link

 

Thanks for the link, but it seem different to me or perhaps I need more pointers how to use it with my case