PHP: Get file extension not working on uploads to S3

I am using the Amazon S3 API to upload files and I am changing the name of the file each time I upload.

So for example:

Dog.png > 3Sf5f.png

Now I got the random part working as such:

function rand_string( $length ) {
            $chars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";  

            $size = strlen( $chars );
            for( $i = 0; $i < $length; $i++ ) {
                $str .= $chars[ rand( 0, $size - 1 ) ];
            }

            return $str;
        }   

So I set the random_string to the name parameter as such:

$params->key = rand_string(5);

Now my problem is that this wont show any extension. So the file will upload as 3Sf5f instead of 3Sf5f.png.

The variable $filename gives me the full name of the file with its extension.

If I use $params->key = rand_string(5).'${filename}'; I get:

3Sf5fDog.png

So I tried to retrieve the $filename extension and apply it. I tried more than 30 methods without any positive one.

For example I tried the $path_info(), I tried substr(strrchr($file_name,'.'),1); any many more. All of them give me either 3Sf5fDog.png or just 3Sf5f.

An example of what I tried:

// As @jcinacio pointed out. Change this to: 
//
//   $file_name = "${filename}";
//
$file_name = '${filename}'  // Is is wrong, since '..' does not evaluate 

$params->key = rand_string(5).$file_name;
=
3Sf5fDog.png

.

$file_name = substr(strrchr('${filename}', '.'), 1);

$params->key = rand_string(5).$file_name;
=
3Sf5f

.

$filename = "example.png"   // If I declare my own the filename it works.
$file_name = substr(strrchr('${filename}', '.'), 1);

$params->key = rand_string(5).$file_name;
=
3Sf5f.png

T