I'm trying to use `array_flip` to print duplicate values in a comma separated format

```
$a=array("a"=>"red","b"=>"green","c"=>"blue","d"=>"blue");
$flip=array_flip($a);
print_r($flip);

```

My output is:

```
Array ( [red] => a [green] => b [blue] => d )

```

But my intended output is:

```
Array ( [red] => a [green] => b [blue] => c,d)

```

How can I get that output?