src/Observable.php
public function concatMapTo(ObservableInterface $observable, callable $resultSelector = null): Observable
    {
        return $this->concatMap(function () use ($observable) {
            return $observable;
        }, $resultSelector);
    }
public function concatMapTo(ObservableInterface $observable, callable $resultSelector = null): Observable
    {
        return $this->concatMap(function () use ($observable) {
            return $observable;
        }, $resultSelector);
    }
public function concatMapTo ( ObservableInterface $ observable , callable $ resultSelector = null ) : Observable { return $ this -> concatMap ( function ( ) use ( $ observable ) { return $ observable ; } , $ resultSelector ) ; }
Projects each element of the source observable sequence to the other observable sequence and merges the
resulting observable sequences into one observable sequence.

@param ObservableInterface $observable - An an observable sequence to project each element from the source
sequence onto.

@param callable $resultSelector A transform function to apply to each element of the intermediate sequence.
The resultSelector is called with the following information:
- the value of the outer element
- the value of the inner element
- the index of the outer element
- the index of the inner element

@return Observable An observable sequence whose elements are the result of invoking the one-to-many
transform function collectionSelector on each element of the input sequence and then mapping each of those
sequence elements and their corresponding source element to a result element.

@demo concat/concatMapTo.php
@operator
@reactivex flatMap
Projects each element of the source observable sequence to the other observable sequence and merges the resulting observable sequences into one observable sequence .