I have written in overall_header.tpl
<?php if ($page_name == "portal.php"): ?>
<div align="center"></div>
<?php else: ?>
<div align="center"><img src="images/my_logo.png" /></a></div>
<?php endif; ?>
<div align="center"></div>
<?php else: ?>
<div align="center"><img src="images/my_logo.png" /></a></div>
<?php endif; ?>
but it doesn't work... any suggestion? Thanks!
---------------------
Solved in this way....

<?php if ($_SERVER['SCRIPT_NAME'] == "/portal.php" ): ?>
<?php else: ?>
<div align="center"><img src="images/my_logo.png" /></a></div>
<?php endif; ?>
<?php else: ?>
<div align="center"><img src="images/my_logo.png" /></a></div>
<?php endif; ?>