Text file - define Placeholder, read out via PHP - generate HTML/CSS on Webside

you need to parse the contents of your file:

<?php
$tokens = array(
  'title'    => array('type' => 'multi-line', 'tag' => 'h1'),
  'break'    => array('type' => 'single',  'tag' => 'br'),
  'headline' => array('type' => 'single',  'tag' => 'hr')
);
$currentToken = null;
// Loop
foreach (file('input.txt') as $line) {
  if(strlen($line)==0)//empty case
    continue;
  //check tags
  if(preg_match('/\[(\w+)\]/', $line, $match)){
    if(isset($tokens[strtolower($match[1])])) {
      //multi-line case
      if($currentToken != null and $currentToken['type'] == 'multi-line') {
        echo "</{$currentToken['tag']}>"; //close multi-line Tag
      }
      $currentToken = $tokens[strtolower($match[1])];
      //single and multi-line
      echo ( $currentToken['type'] == 'single')?
        "\n<{$currentToken['tag']}/>": // print a single tag
        "<{$currentToken['tag']}>"     //open multiline tag
      ;
    }
  } else {
    echo $line;
  }
}

To quote @13ruce1337, you seem to be looking for a template engine. There are many of those in PHP, the most common would be Twig and Smarty.

You could create your own, but it is a complex system, and there are many possibilities for errors, thus I wouldn't suggest building one for regular use until you are confident in your abilities. However, it is an extremely good learning exercise.

Try to learn some Regular Expressions and just write some code.

here is a little example, maybe too weak,but it's a good start:

<?php
$fp = fopen('/tmp/test.txt','r');
while(!feof($fp))
{
    $content = trim(fgets($fp,4096));
    if(!$content) continue;
    //see if this is a tag? if it is, set the tag name;
    if(preg_match('/\[(\w+)\]/', $content,$match))
    {
        $tagname = $match[1];
    }
    else//if it is not a tag,then its the content.
    {
        $tagname = $tagname ? $tagname : 'div';
        echo "<{$tagname}>{$content}</{$tagname}>\n";
    }
}

BUT! to invent a new markup languange is not a good idea,use HTML,or Markdown is a better solution.

if you just want to write html without writing tags, Markdown is a good choice;