WP-Mix

Quick PHP script for automatically selecting the best content type based on what the browser accepts. Works as-is for WordPress, or is easily modified to work with any PHP page.

To implement, add the following to your theme’s functions.php file (or plugin file):

function selctype($ctype) {
    if(preg_match("/wp-admin/",$_SERVER["REQUEST_URI"])) {
	    return "text/html";
    }
    $accept=$_SERVER["HTTP_ACCEPT"];
    if(preg_match("|application/xhtml\+xml(;q=0\.([1-9]+))?|i",$accept,$m)) {
		$q1 = $m[2];
		if($q1){
			$q2 = 10;
			if(preg_match("|text/html;q=0\.([1-9]+)?|i",$accept,$m)) {
     			$q2 = $m[1];
     		}
     		if($q1 >= $q2)
				return "application/xhtml+xml";
			else
				return "text/html";
		}
		return "application/xhtml+xml"; 
    } elseif (preg_match("|text/xml(;q=0\.([1-9]+))?|i",$accept,$m)) {
		$q1 = $matches[2];
		if($q1){
			$q2 = 0;
     		if(preg_match("|text/html;q=0(\.[1-9]+)|i",$accept, $m))
     			$q2 = $matches[1];
     		if($q1 >= $q2)
				return "text/xml";
			else
				return "text/html";
		}
	    return "text/xml";
    }
    return $ctype;
}
add_filter('option_html_type','selctype'); ?>

To modify for use outside of WordPress, simply remove or comment out the last line and include in your web pages.