Avoiding of violating LSP

I want to separate data from source of the data. One class for database interaction and class for data manipulation. But my approach violates LSP: preconditions cannot be strengthened in a subtype and raises strict error: Declaration of DataRepositoryItem::save() should be compatible with DataRepositoryAbstract::save(DataAbstract $data)

class DataAbstract {
}

class DataItem extends DataAbstract {
}

class DataObject extends DataAbstract {
}

abstract class DataRepositoryAbstract {
    /** @return DataAbstract */
    public function loadOne(){}
    /** @return DataAbstract[] */
    public function loadAll(){}                          
    public function save(DataAbstract $data){}
}

class DataRepositoryItem extends DataRepositoryAbstract {
    /** @return DataItem */
    public function loadOne(){}
    /** @return DataItem[] */
    public function loadAll(){}
    public function save(DataItem $data) {}               // <--- violates LSP, how to avoid it?
}

class DataRepositoryObject extends DataRepositoryAbstract {
    /** @return DataObject */
    public function loadOne(){}
    /** @return DataObject[] */
    public function loadAll(){}
    public function save(DataObject $data) {}             // <--- violates LSP, how to avoid it?
}

How to recombine the code to fit LSP?

Update: Ok, I could rewrite methods.

class DataRepositoryItem extends DataRepositoryAbstract {
    /** @return DataItem */
    public function loadOne(){}
    /** @return DataItem[] */
    public function loadAll(){}
    public function save(DataAbstract $data) {
        assert($date instanceof DataItem);
        //...
    }               
}

Works in PHP, but still violates LSP. How to avoid it?

from Avoiding of violating LSP