param_hasher.php

<?php

$params = array(
    "type" => "json",
    "v" => 1,
    "api_key" => "4539653423sa35",
    "topic" => "science",
    "edition" => "fall"
);

# List of keys to ignore
$ignore = array("type", "api_key");

# We have to abstract a keysorter function because
# ksort() sorts by reference, making it unchainable
function keysort($array) {
    ksort($array);
    return $array;
}

# This will show you what the hasher/key maker will do before it serializes/hashes
print_r(keysort(array_diff_key($params, array_fill_keys($ignore, null))));

# This is the hashed key, ready for apc_store(), apc_fetch(), apc_exists(), etc
echo md5(serialize(keysort(array_diff_key($params, array_fill_keys($ignore, null)))));