I downloaded html5 boilerplate and it wouldnt validate with this in the header.
Try to pass it through the web.config or htacess file
Web.Config
<httpProtocol>
<customHeaders>
<clear />
<add name="X-UA-Compatible" value="IE=Edge,chrome=1" />
</customHeaders>
</httpProtocol>
your page will be valid after that. Sorry I am not a php guy.
http://www.validatethis.co.uk/tag/x-ua-compatible/
Aaron Layton has it all hear :) Just Scroll down to the "The fix" and skip all the above :)
Or you could add it to your .htaccess file like this:
<FilesMatch "\.(htm|html|php)$">
<IfModule mod_headers.c>
BrowserMatch MSIE ie
Header set X-UA-Compatible "IE=Edge,chrome=1" env=ie
</IfModule>
</FilesMatch>
How do i test to make sure this is working properly
Make a request to a URI and look at the response headers. There are plenty of tools to do that, including Charles Proxy, Firebug and Chrome Developer Tools.
What does the filesmatch parameter do?
It is described in the manual
The best htaccess configuration that I found is this one below:
<IfModule mod_setenvif.c>
<IfModule mod_headers.c>
BrowserMatch MSIE ie
Header set X-UA-Compatible "IE=Edge,chrome=1" env=ie
</IfModule>
</IfModule>
Because it sends the header just for IE browsers.