$errors = [];
$missing = [];
// check if the form has been submitted
if (isset($_POST['send'])) {
    // email processing script
    $to = 'david@example.com';
    $subject = 'Feedback from Japan Journey';
    // list expected fields
    $expected = ['name', 'email', 'comments', 'subscribe', 'interests', 'howhear', 'characteristics', 'terms'];
    $required = ['name', 'comments', 'email', 'subscribe', 'interests', 'howhear', 'characteristics', 'terms'];

if (!$suspect) {
    foreach ($_POST as $key => $value) {
        // assign to temporary variable and strip whitespace if not an array
        $temp = is_array($value) ? $value : trim($value);
        // if empty and required, add to $missing array
        if (empty($temp) && in_array($key, $required)) {
            $missing[] = $key;
            ${$key} = '';
        } elseif (in_array($key, $expected)) {
            // otherwise, assign to a variable of the same name as $key
            ${$key} = $temp;
        }
    }
}

<input name="email" id="email" type="text"
<?php if ($missing || $errors) {
echo 'value="' . htmlentities($email) . '"';
 } ?>>

$pattern = '/Content-Type:|Bcc:|Cc:/i';

// assume nothing is suspect
$suspect = false;
// create a pattern to locate suspect phrases
$pattern = '/Content-Type:|Bcc:|Cc:/i';

// function to check for suspect phrases
function isSuspect($val, $pattern, &$suspect) {
    // if the variable is an array, loop through each element
    // and pass it recursively back to the same function
    if (is_array($val)) {
        foreach ($val as $item) {
            isSuspect($item, $pattern, $suspect);
        }
    } else {
        // if one of the suspect phrases is found, set Boolean to true
        if (preg_match($pattern, $val)) {
            $suspect = true;
        }
    }
}
// check the $_POST array and any subarrays for suspect content
isSuspect($_POST, $pattern, $suspect);