diff --git a/src/Currency.php b/src/Currency.php
index 00de150..f8ba485 100644
--- a/src/Currency.php
+++ b/src/Currency.php
@@ -38,7 +38,7 @@ public static function set_rate(
 			return self::invalid_currency_code_error( $currency );
 		}
 
-		$option_name = '1984_woo_dk_currency_rate_' . strtolower( $currency );
+		$option_name = '1984_woo_dk_currency_rate_' . mb_strtolower( $currency );
 
 		$float_value = (float) $rate;
 
@@ -62,7 +62,7 @@ public static function get_rate(
 			return self::invalid_currency_code_error( $currency );
 		}
 
-		$option_name = '1984_woo_dk_currency_rate_' . strtolower( $currency );
+		$option_name = '1984_woo_dk_currency_rate_' . mb_strtolower( $currency );
 
 		$rate = get_option( $option_name, 0 );
 
@@ -105,7 +105,7 @@ public static function convert(
 		}
 
 		$from_rate = get_option(
-			'1984_woo_dk_currency_rate_' . strtolower( $from ),
+			'1984_woo_dk_currency_rate_' . mb_strtolower( $from ),
 		);
 
 		if ( ! $from_rate ) {
@@ -124,7 +124,7 @@ public static function convert(
 		}
 
 		$to_rate = get_option(
-			'1984_woo_dk_currency_rate_' . strtolower( $to )
+			'1984_woo_dk_currency_rate_' . mb_strtolower( $to )
 		);
 
 		if ( ! $to_rate ) {
diff --git a/src/Import/ProductVariations.php b/src/Import/ProductVariations.php
index 7820b04..c93854c 100644
--- a/src/Import/ProductVariations.php
+++ b/src/Import/ProductVariations.php
@@ -87,14 +87,16 @@ public static function attributes_to_woocommerce_variation_attributes(
 			return array();
 		}
 
+		$lower_case_code = mb_strtolower( $code );
+
 		$variations = self::get_variations();
 
-		$attributes = $variations[ $code ]->attributes;
+		$attributes = $variations[ $lower_case_code ]->attributes;
 
 		$woocommerce_variation_attributes = array();
 
 		foreach ( $attributes as $code => $attribute ) {
-			$woocommerce_variation_attributes[ strtolower( $code ) ] = array_keys(
+			$woocommerce_variation_attributes[ $lower_case_code ] = array_keys(
 				$attribute->values
 			);
 		}
@@ -313,7 +315,7 @@ public static function get_attribute_from_dk(
 			$code        = $object->CODE;
 			return (object) array(
 				'id'          => $record_id,
-				'code'        => strtolower( $code ),
+				'code'        => mb_strtolower( $code ),
 				'description' => $description,
 				'values'      => self::get_variation_attributes( $record_id ),
 			);
@@ -381,8 +383,8 @@ public static function get_variation_attributes_from_dk(
 			$attributes = array();
 
 			foreach ( $result->data as $a ) {
-				$code     = strtolower( $a->HEADCODE );
-				$code_key = strtolower( $code );
+				$code     = mb_strtolower( $a->HEADCODE );
+				$code_key = mb_strtolower( $code );
 
 				$attributes[ $code ] = (object) array(
 					'code' => $code,
@@ -406,7 +408,7 @@ public static function get_variation_attributes_from_dk(
 	public static function get_attribute_name( string $code ): string {
 		$attribute_values = self::get_attribute_values();
 
-		$key = strtolower( $code );
+		$key = mb_strtolower( $code );
 
 		if ( ! key_exists( $key, $attribute_values ) ) {
 			return $code;
@@ -469,8 +471,8 @@ public static function get_attribute_values_from_dk(): array {
 					$name = $code;
 				}
 
-				$values[ strtolower( $code ) ] = (object) array(
-					'code' => strtolower( $code ),
+				$values[ mb_strtolower( $code ) ] = (object) array(
+					'code' => mb_strtolower( $code ),
 					'name' => $name,
 				);
 			}
@@ -491,7 +493,7 @@ public static function parse_variations_json( array $variation_json ): array {
 
 		foreach ( $variation_json as $vj ) {
 			$variation   = array();
-			$code        = strtolower( $vj->CODE );
+			$code        = mb_strtolower( $vj->CODE );
 			$description = $vj->DESCRIPTION;
 
 			$variation['skus'] = self::get_product_skus_by_variation_from_dk(
@@ -499,18 +501,18 @@ public static function parse_variations_json( array $variation_json ): array {
 			);
 
 			$variations[ $code ] = array(
-				'code'        => strtolower( $code ),
+				'code'        => mb_strtolower( $code ),
 				'description' => $description,
 			);
 
 			if ( property_exists( $vj, 'SUBGROUP1' ) ) {
-				$variation['attributes'][ strtolower( $vj->SUBGROUP1 ) ] =
-				self::get_attribute( strtolower( $vj->SUBGROUP1 ) );
+				$variation['attributes'][ mb_strtolower( $vj->SUBGROUP1 ) ] =
+				self::get_attribute( mb_strtolower( $vj->SUBGROUP1 ) );
 			}
 
 			if ( property_exists( $vj, 'SUBGROUP2' ) ) {
-				$variation['attributes'][ strtolower( $vj->SUBGROUP2 ) ] =
-				self::get_attribute( strtolower( $vj->SUBGROUP2 ) );
+				$variation['attributes'][ mb_strtolower( $vj->SUBGROUP2 ) ] =
+				self::get_attribute( mb_strtolower( $vj->SUBGROUP2 ) );
 			}
 
 			$variations[ $code ] = (object) $variation;
diff --git a/src/Import/Products.php b/src/Import/Products.php
index 9841468..836a9fb 100644
--- a/src/Import/Products.php
+++ b/src/Import/Products.php
@@ -215,15 +215,24 @@ public static function json_to_new_product(
 			return false;
 		}
 
-		if ( strtolower( Config::get_shipping_sku() ) === strtolower( $json_object->ItemCode ) ) {
+		if (
+			mb_strtolower( Config::get_shipping_sku() ) ===
+			mb_strtolower( $json_object->ItemCode )
+		) {
 			return false;
 		}
 
-		if ( strtolower( Config::get_cost_sku() ) === strtolower( $json_object->ItemCode ) ) {
+		if (
+			mb_strtolower( Config::get_cost_sku() ) ===
+			mb_strtolower( $json_object->ItemCode )
+		) {
 			return false;
 		}
 
-		if ( ! $json_object->ShowItemInWebShop && ! Config::get_import_nonweb_products() ) {
+		if (
+			! $json_object->ShowItemInWebShop &&
+			! Config::get_import_nonweb_products()
+		) {
 			return false;
 		}
 
@@ -730,13 +739,13 @@ public static function merge_variations(
 				}
 				$variation = array(
 					'quantity'    => (float) $v->Quantity,
-					'attribute_1' => strtolower( $attribute_names[0] ),
-					'code_1'      => strtolower( $v->Code ),
+					'attribute_1' => mb_strtolower( $attribute_names[0] ),
+					'code_1'      => mb_strtolower( $v->Code ),
 				);
 
 				if ( property_exists( $v, 'Code2' ) ) {
-					$variation['attribute_2'] = strtolower( $attribute_names[1] );
-					$variation['code_2']      = strtolower( $v->Code2 );
+					$variation['attribute_2'] = mb_strtolower( $attribute_names[1] );
+					$variation['code_2']      = mb_strtolower( $v->Code2 );
 				}
 
 				$variations_array[] = (object) $variation;