Don't use numeric IDs for internal PHP serialization.

NOTE: serialize/unserialize is used mainly when cloning Statements.
NOTE: it's unclear if this completely fixes T157442

TODO: proper unit tests, including tests for b/c mode

Bug: T157442

src/Entity/EntityIdValue.php

@@ -31,8 +31,8 @@ public function __construct( EntityId $entityId ) {
 	 */
 	public function serialize() {
 		return json_encode( [
-			$this->entityId->getEntityType(),
-			$this->getNumericId()
+			get_class( $this->entityId ),
+			$this->entityId->getSerialization()
 		] );
 	}
 
@@ -58,10 +58,14 @@ private function getNumericId() {
 	 * @throws IllegalValueException
 	 */
 	public function unserialize( $serialized ) {
-		list( $entityType, $numericId ) = json_decode( $serialized );
+		list( $type, $id ) = json_decode( $serialized );
 
 		try {
-			$entityId = LegacyIdInterpreter::newIdFromTypeAndNumber( $entityType, $numericId );
+			if ( is_string( $id ) ) {
+				$entityId = new $type( $id );
+			} else {
+				$entityId = self::newIdFromTypeAndNumber( $type, $id );
+			}
 		} catch ( InvalidArgumentException $ex ) {
 			throw new IllegalValueException( 'Invalid EntityIdValue serialization.' );
 		}

src/Snak/PropertyValueSnak.php

@@ -53,7 +53,7 @@ public function getDataValue() {
 	 * @return string
 	 */
 	public function serialize() {
-		return serialize( [ $this->propertyId->getNumericId(), $this->dataValue ] );
+		return serialize( [ $this->propertyId->getSerialization(), $this->dataValue ] );
 	}
 
 	/**
@@ -64,8 +64,8 @@ public function serialize() {
 	 * @param string $serialized
 	 */
 	public function unserialize( $serialized ) {
-		list( $numericId, $dataValue ) = unserialize( $serialized );
-		$this->__construct( $numericId, $dataValue );
+		list( $propertyId, $dataValue ) = unserialize( $serialized );
+		$this->__construct( self::newPropertyId( $propertyId ), $dataValue );
 	}
 
 	/**

src/Snak/SnakObject.php

@@ -102,7 +102,7 @@ public function equals( $target ) {
 	 * @return string
 	 */
 	public function serialize() {
-		return serialize( $this->propertyId->getNumericId() );
+		return serialize( $this->propertyId->getSerialization() );
 	}
 
 	/**
@@ -113,7 +113,24 @@ public function serialize() {
 	 * @param string $serialized
 	 */
 	public function unserialize( $serialized ) {
-		$this->propertyId = PropertyId::newFromNumber( unserialize( $serialized ) );
+		$this->propertyId = self::newPropertyId( unserialize( $serialized ) );
+	}
+
+	/**
+	 * @param mixed $unserialized
+	 *
+	 * @return PropertyId
+	 */
+	protected static function newPropertyId( $unserialized ) {
+		if ( is_int( $unserialized ) ) {
+			return PropertyId::newFromNumber( $unserialized );
+		} elseif ( is_string( $unserialized ) ) {
+			return new PropertyId( $unserialized );
+		} elseif ( $unserialized instanceof PropertyId ) {
+			return $unserialized;
+		} else {
+			throw new InvalidArgumentException( 'unexpected property ID serialization' );
+		}
 	}
 
 }