demo.php

<?php

echo Tag::p()->text('Hello world!');
// <p>Hello world!</p>

echo Tag::p()->align('center');
// <p align="center"></p>

$form = Tag::form();
$form->input()
  ->type('search')
  ->name('query');
echo $form;
// <form><input type="search" name="query"></form>

echo Tag::input()->checked(true);
// <input checked>

echo Tag::myAwesomeCustomElement();
// <my-awesome-custom-element></my-awesome-custom-element>

echo Tag::img()->dataSrc('foo.png');
// <img data-src="foo.png">

Tag.php

<?php

/**
 * @method static Tag a()
 * @method static Tag audio()
 * @method static Tag body()
 * @method static Tag br()
 * @method static Tag button()
 * @method static Tag div()
 * @method static Tag form()
 * @method static Tag h1()
 * @method static Tag h2()
 * @method static Tag h3()
 * @method static Tag head()
 * @method static Tag hr()
 * @method static Tag html()
 * @method static Tag iframe()
 * @method static Tag img()
 * @method static Tag input()
 * @method static Tag li()
 * @method static Tag link()
 * @method static Tag meta()
 * @method static Tag ol()
 * @method static Tag p()
 * @method static Tag script()
 * @method static Tag span()
 * @method static Tag table()
 * @method static Tag tbody()
 * @method static Tag td()
 * @method static Tag th()
 * @method static Tag thead()
 * @method static Tag title()
 * @method static Tag tr()
 * @method static Tag ul()
 * @method static Tag video()
 * @method Tag a()
 * @method Tag audio()
 * @method Tag body()
 * @method Tag br()
 * @method Tag button()
 * @method Tag div()
 * @method Tag form()
 * @method Tag h1()
 * @method Tag h2()
 * @method Tag h3()
 * @method Tag head()
 * @method Tag hr()
 * @method Tag html()
 * @method Tag iframe()
 * @method Tag img()
 * @method Tag input()
 * @method Tag li()
 * @method Tag link()
 * @method Tag meta()
 * @method Tag ol()
 * @method Tag p()
 * @method Tag script()
 * @method Tag span()
 * @method Tag table()
 * @method Tag tbody()
 * @method Tag td()
 * @method Tag th()
 * @method Tag thead()
 * @method Tag title()
 * @method Tag tr()
 * @method Tag ul()
 * @method Tag video()
 * @method Tag checked(bool $value)
 * @method Tag class(string $value)
 * @method Tag disabled(bool $value)
 * @method Tag height(string $value)
 * @method Tag hidden(bool $value)
 * @method Tag href(string $value)
 * @method Tag id(string $value)
 * @method Tag loading(string $value)
 * @method Tag name(string $value)
 * @method Tag placeholder(string $value)
 * @method Tag selected(bool $value)
 * @method Tag src(string $value)
 * @method Tag style(string $value)
 * @method Tag title(string $value)
 * @method Tag value(string $value)
 * @method Tag width(string $value)
 */
class Tag
{
  private $name;
  private $attributes;
  private $children;

  function __construct($name)
  {
    $this->name = self::camelToKebab($name);
    $this->attributes = [];
    $this->children = [];
  }

  function html($value)
  {
    $this->children[] = $value;
    return $this;
  }

  function text($value)
  {
    $this->children[] = htmlspecialchars($value);
    return $this;
  }

  function __call($name, $arguments)
  {
    if (empty($arguments)) {
      $child = new Tag($name);
      $this->children[] = $child;
      return $child;
    } else {
      $name = self::camelToKebab($name);
      $value = $arguments[0];
      if (is_bool($value)) {
        $this->attributes[$name] = $value;
      } else {
        $this->attributes[$name] = htmlspecialchars($arguments[0]);
      }
      return $this;
    }
  }

  function __toString()
  {
    global $__tag_void;
    $tag = "<$this->name";
    foreach ($this->attributes as $name => $value) {
      if (is_bool($value)) {
        if ($value) {
          $tag .= " $name";
        }
      } else {
        $tag .= " $name=\"$value\"";
      }
    }
    $tag .= '>';
    if (in_array($this->name, $__tag_void)) {
      return $tag;
    }
    foreach ($this->children as $child) {
      $tag .= $child;
    }
    $tag .= "</$this->name>";
    return $tag;
  }

  static function __callStatic($name, $args)
  {
    return new Tag($name);
  }

  private static function camelToKebab($string)
  {
    return strtolower(preg_replace('/[A-Z]/', '-$0', $string));
  }
}

$__tag_void = [
  'area',
  'base',
  'br',
  'col',
  'embed',
  'hr',
  'img',
  'input',
  'link',
  'meta',
  'param',
  'source',
  'track',
  'wbr'
];