Basic Php form validation

<?php
function user_form_validate(array $input)
{
    $errors = [];
    if($input['email'] == '') {
        $errors['email'][] = 'Email address is required.';
    } elseif(filter_var($input['email'], FILTER_VALIDATE_EMAIL) === false) {
        $errors['email'][] = 'Email address must be valid.';
    }

    return $errors;
}

function user_form_html_render(
        $action = '',
        array $input,
        array $errors
    )
{
    ?>
    <form method='POST' action="<?= $action ?>">
        <label for="email">Email address:</label>
        <input type="text" name="email" id="email" value=
        "<?= html_escape($input['email']) ?>">
        <?=  html_list($errors['email']) ?>
        <input type="submit">
    </form>
    <?php
}

function html_escape($string) {
    return htmlspecialchars($string);
};

function html_list($messages) {
    return
        empty($messages) || !is_array($messages)
        ? ''
        : '<ul><li>' .
        implode('</li><li>', array_map('html_escape', $messages)) .
        '</li></ul>';
};

$input  = ['email' => null];
$errors = $input;

if($_SERVER['REQUEST_METHOD'] == 'POST') {
    $filtered = array_filter($_POST, 'is_string');
    $filtered = array_map('trim', $filtered);
    foreach($input as $k => $v)
        if(isset($filtered[$k]))
            $input[$k] = $v;
    $errors   = user_form_validate($input);
    $valid    = empty(array_filter($errors));

    if($valid) {
        // Form values look good, do what you want with values.
        echo 'Email address entered is: ' . $input['email'];
    } else {
        // Display form with data and errors
        user_form_html_render('', $input, $errors);
    }
} else {
    user_form_html_render('', $input, $errors);