<?php
$arr = array(
  "pērkons" => 1,
  "saule" => 1,
  "zeme" => 1,
  "zobens" => 1
);

echo isset($arr['saule'])? "ir" : "nav";
echo "\n";

echo isset($arr['koks'])? "ir" : "nav";
echo "\n";

echo isset($arr['zeme'])? "ir" : "nav";
echo "\n";

$arr['koks']=1;

echo isset($arr['koks'])? "ir" : "nav";
echo "\n";

foreach($arr as $key=>$val){
  echo $key." ";
}

ir
nav
ir
ir
pērkons saule zeme zobens koks