Why should we separate PHP from HTML

Question

I'm rather new to programming and i know how to separate PHP from HTML, but i would like to know if there is any difference in doing

this:

<?php $rand="I love apples" ?>
<h1>This is a title</h1>
<div>
    <p>This is a paragraph</p>
    <?php echo"The variable contains the string $rand"; ?>
</div>
?>

compared to doing this:

<?php
    echo "<h1>This is a title</h1>";
    echo "<div>";
    echo "<p>This is a paragraph</p>";
    echo "The variable contains the string $rand";
    echo "</div>";
?>

Is there any difference between in performance etc, between splitting the PHP code from the HTML code and just echoing the whole page in php?

Answer 1

<?php $rand="I love apples" ?>
<h1>This is a title</h1>
<div>
    <p>This is a paragraph</p>
    <?php echo"The variable contains the string $rand"; ?>
</div>
?>

The above looks poorly separated. This is what php/html separation should look like:

<?php 
$rand="I love apples"; 
?>
<h1>This is a title</h1>
<div>
    <p>This is a paragraph</p>
    <p>The variable contains the string <?=$rand ?></p>
</div>

Performance-wise, that's not an issue but it would do much favor for programmers to be able to read the code easily, hence the need for HTML/PHP separation practices. Ideally, if you're going to do just one script, keep all your PHP code at top. Also, other reason for the separation is that IDE editors can easily format HTML nicely. If there's a HTML tag inside the PHP tag that is ending with a HTML tag outside of PHP, then HTML cannot be formatted correctly. For example:

<div><p>And it offers so much <?php echo "$features</p>
<h2>Proven Ideas";?></h2>
<p>More details ahead</p>  
</div>

The above will run just fine but the IDE html formatter will likely be confused with missing end tags and won't format making it more difficult for programmers to read them.