Random Logo PHP - Security Issue?

[mdb]

Hey guys,
I'm trying to insert a random logo code into my header template (using Zingas awesome PHP in Templates plugin) but when trying to save the template, MyBB gives me an error about the code being a potential security risk.

Here is the code i'm using:

Code:

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<?php $Img1 = "{$theme['logo']}"; $Img2 = "/images/theme/logo2.png"; $num = rand (1,2); $Image = ${'Img'.$num}; echo "<a href=\""{$mybb->settings['bburl']}/portal.php"\"><img src=\"".$Image."\" title=\""{$mybb->settings['bbname']}"\" width=\""700"\" height=\""150"\" alt=\""{$mybb->settings['bbname']}"\" /</a>"; ? >

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Does anyone know how I could fix that?