Sanitize output of html, remove html, css, js comments and trim output.

sanitize_output.php

<?php

/*
 * Compressing the HTML Output
 */
function sanitize_output($buffer) 
{
    $search = array(
        '/\>[^\S ]+/s',      // strip whitespaces after tags, except space
        '/[^\S ]+\</s',      // strip whitespaces before tags, except space
        '/(\s)+/s',          // shorten multiple whitespace sequences
        '/<!--(.|\s)*?-->/', // Remove HTML comments
		'/\>\s+\</',         // Remove Extra space between Tags
    );

    $replace = array(
        '>',
        '<',
        '\\1',
        '',
		'><',
    );

    $buffer = preg_replace($search, $replace, $buffer);

    return $buffer;
}

// Now put this statement before starting the output
ob_start("sanitize_output");