<?php
$a= Array
(
    "RT III" => 3,
    "RT II" => 2,
    "RT I" => 2,
    "RT V" => 1,
    "RT IV" => 1
);
foreach ($a as $key => $value) {
	if($value==next($value)){
		$c= next($key);
		$h = implode(",", $c);
	}else{
		$h = $key;
	}
	$r[] = $h;
}

echo "<pre>";
        print_r($a);
    echo"</pre>";
    echo "<pre>";
        print_r($r);
    echo"</pre>";
?>