Strange expressions in PHP

Question

Someone explains why this:

<?php

   $world = _("World"); 
   $str = <<<EOF
   <p>Hello</p>
   <p>$world</p>
EOF;
    echo $str;

?>

generates this:

Hello

World

error-free?

what would that be <<<, EOF, <p>$world</p>, _("World");. Finally I wanted to understand what these things are that I’ve never seen developing in everyday life PHP

We can define anything after <<< EOF?

It would be right to develop PHP with html using this? if yes why? if not why ?

Example, instead:

echo "<table><tr><td>".$titulo."</td></tr></table>";

or so:

<table><tr><td><?php echo $titulo ?></td></tr></table>

Write that:

//tabela
$minha_tabela = <<<EOF
<table>
 <tr>
  <td>titulo e $titulo</td>
 <tr>
<table>    

EOF;
//apenas dando esse echo mostro minha tabela aonde eu quero
echo $minha_tabela;