src/Observable.php
public function concatMap(callable $selector, callable $resultSelector = null): Observable
    {
        return $this->lift(function () use ($selector, $resultSelector) {
            return new ConcatMapOperator(new ObservableFactoryWrapper($selector), $resultSelector);
        });
    }
public function concatMap(callable $selector, callable $resultSelector = null): Observable
    {
        return $this->lift(function () use ($selector, $resultSelector) {
            return new ConcatMapOperator(new ObservableFactoryWrapper($selector), $resultSelector);
        });
    }
public function concatMap ( callable $ selector , callable $ resultSelector = null ) : Observable { return $ this -> lift ( function ( ) use ( $ selector , $ resultSelector ) { return new ConcatMapOperator ( new ObservableFactoryWrapper ( $ selector ) , $ resultSelector ) ; } ) ; }
Projects each element of an observable sequence to an observable sequence and concatenates the resulting
observable sequences into one observable sequence.

@param callable $selector A transform function to apply to each element from the source sequence onto.
The selector is called with the following information:
- the value of the element
- the index of the element
- the Observable object being subscribed

@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/concatMap.php
@operator
@reactivex flatMap
Projects each element of an observable sequence to an observable sequence and concatenates the resulting observable sequences into one observable sequence .