The chapter 'Add a reference to a statement' has a mistake:
foreach( $statementList->getByPropertyId( PropertyId::newFromNumber( 99 ) )->getIterator() ) {
if( $statement->getReferences()->isEmpty() ) {
$statement->addNewReference( $referenceSnaks );
}
}should be with '$statement' in the foreach declaration
foreach( $statementList->getByPropertyId( PropertyId::newFromNumber( 99 ) )->getIterator() as $statement) {
if( $statement->getReferences()->isEmpty() ) {
$statement->addNewReference( $referenceSnaks );
}
}