It's not that hard. Actually if all you want to do is add some html code then there is no need to know php just add your html code without the <head> or <body> tags. As quoted from the manual,
Warning: you mustn't include full html pages that contain an html header or footer (tags like <head> or <body>), nor can the included file do file header manipulation, e.g. reading another (non-coppermine) cookie.
To give you an idea, here is the code that generates the blue header you see at the top of the forum. As you can see it's just html code (including a lot of styling).
<div style="width:100%;background-color:#0871A5;">
<table border="0" cellspacing="0" cellpadding="20">
<tr>
<td style="margin-top:5px;margin-bottom:5px;margin-left:5px;margin-right:5px;padding-top:5px;padding-bottom:5px;padding-left:0px;padding-right:0px;border-width:1px;border-style:solid;border-color:#0871A5;font-size:9px;"><a href="/" style="color:#FFFFFF;text-decoration:none;margin-left:5px;margin-right:5px;" title="Home"><img src="/images/cpg.gif" width="88" height="19" border="0" alt="" title="Home" /></a></td>
<td style="margin-top:5px;margin-bottom:5px;padding-top:5px;padding-bottom:5px;padding-left:0px;padding-right:0px;border-width:1px;border-style:solid;border-color:#0871A5;font-size:9px;"><a href="http://forum.coppermine-gallery.net/" style="color:#FFFFFF;text-decoration:none;margin-left:5px;margin-right:5px;" onmouseover="this.style.color='#FFCC66';" onmouseout="this.style.color='#FFFFFF';" title="Support Forum">Support Forum</a></td>
<td style="margin-top:5px;margin-bottom:5px;margin-left:5px;margin-right:5px;padding-top:5px;padding-bottom:5px;padding-left:0px;padding-right:0px;border-width:1px;border-style:solid;border-color:#0871A5;font-size:9px;"><a href="http://sourceforge.net/projects/coppermine/" style="color:#FFFFFF;text-decoration:none;margin-left:5px;margin-right:5px;" onmouseover="this.style.color='#FFCC66';" onmouseout="this.style.color='#FFFFFF';" title="sourceforge project pages">Project</a></td>
<td style="margin-top:5px;margin-bottom:5px;margin-left:5px;margin-right:5px;padding-top:5px;padding-bottom:5px;padding-left:0px;padding-right:0px;border-width:1px;border-style:solid;border-color:#0871A5;font-size:9px;"><a href="http://sourceforge.net/project/showfiles.php?group_id=89658" style="color:#FFFFFF;text-decoration:none;margin-left:5px;margin-right:5px;" onmouseover="this.style.color='#FFCC66';" onmouseout="this.style.color='#FFFFFF';" title="Downloads">Downloads</a></td>
<td style="margin-top:5px;margin-bottom:5px;margin-left:5px;margin-right:5px;padding-top:5px;padding-bottom:5px;padding-left:0px;padding-right:0px;border-width:1px;border-style:solid;border-color:#0871A5;font-size:9px;"><a href="http://coppermine-gallery.net/demo/cpg14x/docs/faq.htm" style="color:#FFFFFF;text-decoration:none;margin-left:5px;margin-right:5px;" onmouseover="this.style.color='#FFCC66';" onmouseout="this.style.color='#FFFFFF';" title="Frequently Asked Questions">FAQ</a></td>
<td style="margin-top:5px;margin-bottom:5px;margin-left:5px;margin-right:5px;padding-top:5px;padding-bottom:5px;padding-left:0px;padding-right:0px;border-width:1px;border-style:solid;border-color:#0871A5;font-size:9px;"><a href="http://coppermine-gallery.net/demo/cpg14x/docs/index.htm" style="color:#FFFFFF;text-decoration:none;margin-left:5px;margin-right:5px;" onmouseover="this.style.color='#FFCC66';" onmouseout="this.style.color='#FFFFFF';" title="Documentation">Documentation</a></td>
<td style="margin-top:5px;margin-bottom:5px;margin-left:5px;margin-right:5px;padding-top:5px;padding-bottom:5px;padding-left:0px;padding-right:0px;border-width:1px;border-style:solid;border-color:#0871A5;font-size:9px;"><a href="http://coppermine-gallery.net/team/" style="color:#FFFFFF;text-decoration:none;margin-left:5px;margin-right:5px;" onmouseover="this.style.color='#FFCC66';" onmouseout="this.style.color='#FFFFFF';" title="about: the coppermine team">About</a></td>
<td style="margin-top:5px;margin-bottom:5px;margin-left:5px;margin-right:5px;padding-top:5px;padding-bottom:5px;padding-left:0px;padding-right:0px;border-width:1px;border-style:solid;border-color:#0871A5;font-size:9px;"><a href="http://coppermine-gallery.net/Obsolete" style="color:#FFFFFF;text-decoration:none;margin-left:5px;margin-right:5px;" onmouseover="this.style.color='#FFCC66';" onmouseout="this.style.color='#FFFFFF';" title="see Coppermine live">Demo</a></td>
<td style="margin-top:5px;margin-bottom:5px;margin-left:5px;margin-right:5px;padding-top:5px;padding-bottom:5px;padding-left:0px;padding-right:0px;border-width:1px;border-style:solid;border-color:#0871A5;font-size:9px;"><a href="http://coppermine-gallery.net/tutorial/" style="color:#FFFFFF;text-decoration:none;margin-left:5px;margin-right:5px;" onmouseover="this.style.color='#FFCC66';" onmouseout="this.style.color='#FFFFFF';" title="Tutorials">Tutorials</a></td>
</tr>
</table>
</div>
↑↑↑<span style="font-size:9px">Custom header include (new) - use it to include your site's navigation into coppermine.</span>↑↑↑
Try it, it can't hurt and if it doesn't work out, then remove the link the congif settings. You won't learn if you don't try.