I would like to know whether I applied the SOLID principle and the repository pattern the right way.My code should derive exchange rates from a database and do basic currency conversions.The database table is very easy. I assume I have a base currency and all exchange rates are against the base currency. So the database table exchangerate is just like this:CREATE TABLE exchangerate (currency varchar(3) PRIMARY_KEY,    exchangerate INT NOT NULL);I also have set the base currency as a global variable (actually it could be a constant too):$GLOBALS[basecurrency] = EUR;I have a currency repository class that talks to the database. I have intentionally not used an ORM (after a months of review of different ORMs, I think, I will still code my own SQL queries). And I have intentionally not used an interface, because I cannot think of any reason why I ever would need another implementation of the currency repository than just fetching the rates from the database. Of course both decisions can be criticised, I would be interested in feedback on this. My reason for not using an ORM is because I have highly efficient SQL queries that have 50 lines. I am just not keen to rewrite them into some ORM logic just to find out, that the performance is horrible:<?phpnamespace Bjc\Iis\Core;use Bjc\Iis\Core\Db;class currencyrepository {    public $rows;    function getAll() {        $db = new Db();        $db->query('SELECT currency, exchangerate FROM exchangerate');        $rows = $db->resultset();        return $rows;    }}I have a class that will serve the exchange rates. This is my data object. I planned to have all exchange rates in one object, so I set this to a singleton because there is no need to have several instances of this class. I would also like to have feedback if it is a wise decision to hold all the exchange rates in one object and if the use of a singleton is correct here.The data object also incorporates some business logic, because if the requested target and source currency both are not the base currency, the class will compute an artificial exchange rate between those too:<?phpnamespace Bjc\Iis\Core;class currencyconversionrate {    private final function __clone() { }    public final function __sleep() {        throw new Exception('Serializing of Singletons is not allowed');    }    static private $instance = null;    public function __construct() {        $currencyrepository = new currencyrepository();        $rows_get_exchangerate = $currencyrepository->getAll();         foreach ($rows_get_exchangerate as $row) {            $currency = $row[currency];            $conversionratearray[$waehrung] = $row[kurs];        } // ende foreach ($rows as $row)        $this->conversionratearray = $conversionratearray;    } // ende function __construct()public static function getInstance() {       if (self::$instance === null) self::$instance = new self;       return self::$instance;    }    public function checkCurrency($currency) {        foreach ($this->conversionratearray as $checkcurrency => $checkconversionrate) {             if ($checkcurrency == $currency) {                $checked = 1;             } // ende if ($checkcurrency == $currency)        } // ende if (foreach ($this->conversionratearray ...)        if ($currency == $GLOBALS[basecurrency]) {            $checked = 1;        } // ende if ($currency == $GLOBALS[basecurrency])        if ($checked != 1) {            throw new Exception('No valid currency.');        } // ende if ($checked != 1)    } // ende function checkCurrency($currency)    public function getConversionrate($sourcecurrency,$targetcurrency) {        try {            $this->checkCurrency($sourcecurrency);            $this->checkCurrency($targetcurrency);            if ($targetcurrency == $sourcecurrency) {                $conversionrate = 1;            } elseif ($GLOBALS[basecurrency] == $targetcurrency) {                $conversionrate = 100 / $this->conversionratearray[$sourcecurrency];            } elseif ($GLOBALS[basecurrency] == $sourcecurrency) {                $conversionrate = $this->conversionratearray[$targetcurrency];            } else {                $conversionrate = (100 / $this->conversionratearray[$sourcecurrency]) * $this->conversionratearray[$targetcurrency];            } // ende if ($this->targetcurrency == $this->sourcecurrency)        } catch (Exception $e) {            $GLOBALS[htmloutput] .= $e->getMessage();            $conversionrate = 0;        } // ende try/catch        return $conversionrate;    } // ende function getConversionrate()} // ende class currencyconversionrateAnd finally I have a class that does the conversion of a certain amount:<?phpnamespace Bjc\Iis\Core;class currencyconvertamount {    public function setAmount($amount) {        $this->amount = $amount;    }    public function setSourceCurrency($sourcecurrency) {        $this->sourcecurrency = $sourcecurrency;    }    public function setTargetCurrency($targetcurrency) {        $this->targetcurrency = $targetcurrency;    }    public function convert() {        $currencyhandler = currencyconversionrate::getInstance();        $conversionrate = $currencyhandler->getConversionrate($this->sourcecurrency,$this->targetcurrency);        $convertedamount = round($this->amount * $conversionrate,2);        return $convertedamount;    } // ende public function convert()} // ende class currencyconvertamountAm I doing correct OOP here?
Derive currency conversion rates from database, keep it available in a class
php;object oriented;repository