foreachループの各コンポーネントで何が起こっているのかを推測します。

foreach（$ validation as $ key => $ value）{

            if（$ validation [$ key] === 1）{
                $ data [$ key] = filter_var（$ data [$ key]、filter_sanitize_email）;
            } elseif（$ validation [$ key] === 3）{
                $ data [$ key] = preg_replace（ '/[^\ p {l}]/u'、 ''、$ data [$ key]）;
                $ generalSanitization = false;
            } elseif（$ validation [$ key] === 4）{
                $ data [$ key] = preg_replace（ '/[^0-9]/'、 ''、$ data [$ key]）;
                $ generalSanitization = false;
            } elseif（$ validation [$ key] === 5）{
                $ data [$ key] = preg_replace（ '/[^\ p {l} \ p {nd} \ s]/u'、 ''、$ data [$ key]）;
                $ generalSanitization = false;
            } elseif（$ validation [$ key] === 6）{
                $ data [$ key] = preg_replace（ '/[^\ p {l} \ p {nd} \ s [：punct：]¿]/u'、 ''、$ data [$ key]）;
            } elseif（$ validation [$ key] === 7）{
                $ data [$ key] = preg_replace（ '/[^\ p {l} \ p {nd} \ s [：punct：]¿\ x20- \ x7f]/u'、 ''、$ data [$ key] ）;
            } elseif（$ validation [$ key] === 8）{
                $ data [$ key] = preg_replace（ '/[^\ x20- \ x7f]/'、 ''、$ data [$ key]）;
            } elseif（$ validation [$ key] === 9）{
                $ data [$ key] = preg_replace（ '/^（[a-za-z0-9 _-]+）（[^a-za-z0-9 _-]+）。*$/'、 '$ 1 '、$ data [$ key]）;
            }

            if（$ generalSanitization === true）{
                $ data [$ key] = sanitization :: string（$ data [$ key]、$ databaseconnection）;
            }
        }
Infiere lo que está pasando en cada uno de los componentes del bucle foreach:

foreach ($validation as $key => $value) {

            if ($validation[$key] === 1) {
                $data[$key] = filter_var($data[$key], FILTER_SANITIZE_EMAIL);
            } elseif ($validation[$key] === 3) {
                $data[$key] = preg_replace('/[^\p{L} ]/u', '', $data[$key]);
                $generalSanitization = false;
            } elseif ($validation[$key] === 4) {
                $data[$key] = preg_replace('/[^0-9]/', '', $data[$key]);
                $generalSanitization = false;
            } elseif ($validation[$key] === 5) {
                $data[$key] = preg_replace('/[^\p{L}\p{Nd}\s]/u', '', $data[$key]);
                $generalSanitization = false;
            } elseif ($validation[$key] === 6) {
                $data[$key] = preg_replace('/[^\p{L}\p{Nd}\s[:punct:]¡¿]/u', '', $data[$key]);
            } elseif ($validation[$key] === 7) {
                $data[$key] = preg_replace('/[^\p{L}\p{Nd}\s[:punct:]¡¿\x20-\x7F]/u', '', $data[$key]);
            } elseif ($validation[$key] === 8) {
                $data[$key] = preg_replace('/[^\x20-\x7F]/', '', $data[$key]);
            } elseif ($validation[$key] === 9) {
                $data[$key] = preg_replace('/^([a-zA-Z0-9_-]+)([^a-zA-Z0-9_-]+).*$/', '$1', $data[$key]);
            }

            if ($generalSanitization === true) {
                $data[$key] = Sanitization::string($data[$key], $databaseConnection);
            }
        }