print_r($InformationRetrieval['KeywordExtraction'][1]);

<?php
$a = array("google" => 15, "search" => 18, "or" => 2, "web" => 4, "is" => 4, "a" => 5, "engine" => 2);

foreach($a as $key=>$value) {
   echo $key;
}
?>

print_r(array_keys($InformationRetrieval['KeywordExtraction'][2]));