gistfile1.php

<?php

$errors = array();
foreach (array('email', 'name', 'age', 'occupation') as $key) {
    if (!isset($_GET[$key]) || empty($_GET[$key])) {
        $errors[] = "You need to fill in $key";
    }
}

gistfile2.php

<?php

function validateArray(array $keys, array $subject)
{
    $errors = array();
    foreach ($keys as $key) {
        if (!isset($subject[$key]) || empty($subject[$key])) {
            $errors[] = $key;
        }
    }
    return $errors;
}

// Now you can do.
foreach (validateArray(array('email', 'name', 'age', 'occupation'), $_GET) as $key) {
  echo "You need to fill in $key";
}