<?php

$data = ["fire", "water", "wind", "fire", "thunder"];

function deck($data){
    $counts = [];
    foreach($data as $elem){
        if(isset($counts[$elem])){
            $counts[$elem]++;
        }else{
            $counts[$elem]=1;
        }
    }

    $max = 0;
    $result = [];

    foreach($counts as $elem => $count){
        if($count < $max){
            continue;
        }
        if($count > $max){
            $result = [];
            $max = $count;
        }
        $result[] = $elem;
    }
    return $result;
}

echo implode(",", deck($data));