Code for the Youtube Tutorial https://youtu.be/g-rUVNo0FWc

Template.php

<?php

function render(string $path, array $data) {

    if (defined('TEMPLATE_DIR')) {
        $path = TEMPLATE_DIR . DIRECTORY_SEPARATOR . $path;
    }
    if (!is_file($path)) {
        trigger_error('Datei "' . $path . '" wurde nicht gefunden');
        return null;
    }
    _templateData($data);
    extract($data, EXTR_SKIP);

    ob_start();
    require_once $path;
    $content = ob_get_clean();
    return $content;
}

function escape(string $data) {
    return htmlspecialchars($data, ENT_QUOTES, 'UTF-8');
}

function _templateData(array $data = null) {
    static $value = [];
    if ($data) {
        return $value = $data;
    }
    return $value;
}

function layout($path = null) {
    static $layoutPath = '';

    if ($path && !(bool) $layoutPath) {
        return $layoutPath = $path;
    }

    $data = _templateData();
    echo render($layoutPath, $data);
    return null;
}

function section($name) {
    static $sections = [];
    if (!isset($sections[$name])) {
        return $sections[$name] = ob_start();
    }
    $content = trim(ob_get_clean());
    $data = _templateData();
    $data[$name] = $content;
    _templateData($data);
    unset($sections[$name]);
    return true;
}