Untitled

I understand that i need to use <script type="text/php"> for the php code for it to be a correct procedure, but at the same time I don’t need access the DOM etc.
I just need my php to work properly with <?php ?> tags .
The problem that I’m facing is that I get white blank page after clicking the links in the example below , its doesn’t happen in a logical order , sometimes I get the blank white page at the first click other times maybe after 3 or 4 clicks.
I have a more complex project started and this bug is killing me.
What is causing this and how can I avoid it.

Thanks ( coffee and beers on me :) )

<html>
<head>
</head>
<body style="background-color:#1c1c1c;margin:0">
<div style="border-top:1px solid #404040">
<div style="color:#fff;;padding:10px">
<a href="index.php?pag=reservas">Welcome</a> to <a href="index.php?pag=inicio">Titanium</a>
<br>
<?php
echo $_GET['pag'];
 ?>
</div>
</div>
</body>
</html>