function getConvertion($currency,$value){
	$conversionRates = array(
		'£'=>'1.0',
		'$'=>'1.59030',
		'€'=>'1.14276'
	);
	return $value * $conversionRate[$currency];
}
function splitRequestString($string){
	$currency = substr($string,0,1));
	$value = explode(' ',$string);
	$value = substr($value[1],0,1)

	return array($currency,$value);
}

$request = splitRequestString($_POST['request']);
$deduct = getConvertion($request[1]);

$update = mysql_query('UPDATE members SET points = (points - '.$deduct.') WHERE id = "'.$_POST['id'].'"')
or die(mysql_error());