What does <? php echo ("<pre>"); ..... echo("</pre>"); ?> mean?

It is nor php nor html it sounds like specific xml tag.

I think you're talking about <pre></pre>. element is displayed in a fixed-width font, and it preserves both spaces and line breaks.

try printing an array with a **<pre>** and whitout **<pre>**

$arr = array(1, 2, 3);

echo '<pre>';
print_r($arr);
echo '</pre>';

print_r($arr); 

echo (""); is a php code, and <prev> tries to be HTML, but isn't.

As @pekka said, its probably supposed to be <pre>

The <prev> tag doesn't exist, but it's probably the <pre> HTML tag to put around debug output, to improve readability. It's not a secret PHP hack. :)