Set cache control headers and manage http cache logic

header_cache.php

/**
 * Set cache control headers and manage http cache logic.
 *
 * @param (int|bool) $expires: true - use cache, false - no cache, int - expires timeout
 * @param (time)     [$modified]: Last modification time stamp
 * @param (string)   [$etag]: ETag content identifier
 * @param (int)      [$die]:  0 - no die, 1 - die on 304, -1 - no 304 headers
 *
 * @return (mixed) status code if cache, false if headers sent, true otherwise
 *
 * @author Dumitru Uzun
 */
function header_cache($expires=true, $modified=NULL, $etag=NULL, $die=true, $cc=NULL) {
    if(headers_sent()) return false;

    defined('DATE_HTTP') or define('DATE_HTTP', 'D, d M Y H:i:s T');

    $time = time();
    header("Date: ".gmdate(DATE_HTTP, $time));

    // Cache-Control: no-cache,max-age=0
    // Pragma: no-cache
    $hcc = get_env('HTTP_CACHE_CONTROL');
    $hpg = get_env('HTTP_PRAGMA');
    if($hcc || $hpg) {
        $t = explode(',', $hcc.','.$hpg);
        $rcc = array(); // request Cache-Control
        foreach($t as $v) if($v = trim($v)) {
            $v = explode('=', $v, 2);
            if(count($v) == 2) {
                $rcc[$v[0]] = trim($v[1]);
            }
            else {
                $v = reset($v);
                $rcc[$v] = $v;
            }
        }
        if(!empty($rcc['no-cache'])) {
            $expires = false;
        }
        elseif($expires && @$rcc['max-age'] == '0') {
            $expires = 0;
        }
        unset($t, $v);
    }

    if(is_int($expires) || $expires) {
        isset($cc) or $cc = defined('CACHE_CONTROL') ? CACHE_CONTROL : 'public';
        $cc and header("Pragma: $cc");
        if(is_int($expires)) {
            // isset($prot) or $prot = get_env('SERVER_PROTOCOL') or $prot = 'HTTP/1.0';
            header('Expires: ' . gmdate(DATE_HTTP, $time+$expires)); // HTTP/1.0
            $cc and $cc = ' ' . $cc . ',';
            header("Cache-Control:$cc max-age=$expires");            // HTTP/1.1
        }

        $fresh = NULL; // NULL - undefined, true - fresh, false - old
        if( $modified > 0 ) {
            header('Last-Modified: '.gmdate(DATE_HTTP, $modified));
            if($fresh !== false ) {
                if($ms = get_env('HTTP_IF_MODIFIED_SINCE')) {
                    $fresh = ($modified <= strtotime($ms));
                } // else - first visit or not suported
            }
        }
        if( $etag ) {
            header('ETag: '.$etag);
            if($ms = get_env('HTTP_IF_NONE_MATCH') or $fresh) {
                $fresh = ($ms == $etag);
            } // else - first visit or not suported
        }

        if(isset($fresh)) {
            if($fresh === true) {
                $code = 304;
                if($die >= 0) {
                    $msg = 'Not Modified';
                    if(strncmp(php_sapi_name(), 'cgi', 3) == 0) {
                        header("Status: $code $msg", true, $code);
                    }
                    else {
                        isset($prot) or $prot = get_env('SERVER_PROTOCOL') or $prot = 'HTTP/1.1';
                        header("$prot $code $msg", true, $code);
                    }
                    if($die) {
                        while(ob_get_level() > 0) ob_end_clean();
                        flush();
                        die();
                    }
                }
                return $code;
            }
            else {
                return 200;
            }
        }
    }
    else {
        header('Pragma: no-cache');
        header('Expires: 0');
        header('Cache-Control: private, no-cache, no-store, must-revalidate, max-age=0');
    }
   return true;
}

// A helper function to get an environment/server variable
function get_env($var) {
    if(func_num_args() > 1) {
        $t = func_get_args();
        return get_env($t);
    }
    if(is_array($var)) {
        foreach($var as $v) {
            $t = get_env($v);
            if($t !== false) return $t;
        }
    }
    else {
        $t = getenv($var);
        if($t === false && array_key_exists($var, $_SERVER)) {
            $t = $_SERVER[$var];
        }
    }
    return $t;
}