Im missing newline with bbCode

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PHP

I use bbCode function to enable text formatting on my website. The problem is that I get problems with newline. Sometimes it creates

<p></p>

without reason, and sometimes I dont get any newlines at all...
To much newline: http://checkoff.org/index.php?page=articles&action=view_article&article_id=8[^]

No newline: http://checkoff.org/index.php?page=articles&action=view_article&article_id=6[^]

This is my code:

<?php 
$get_article_text = bbcode($get_article_text);
$get_article_text = str_replace("{linebreak}", "\n", $get_article_text);

echo"
<h1>$get_article_title</h1>

<p>$get_article_text</p>
";
?>

And my bbCode function:
http://pastebin.com/xhGKrY3f[^]