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I have cacheable dynamic content made in PHP 5.1.0+. I already send the correct headers (including Last-Modified and ETag) to clients.

I now want my script to be able to answer $_SERVER['HTTP_IF_MODIFIED_SINCE'] and $_SERVER['HTTP_IF_NONE_MATCH'] when present. When the conditions matches, I want to answer a HTTP 304 "Not Modified" to clients.

What are the correct conditions? When exactly I issue a 304 instead of the whole content?

The accepted answer in question How to know when to send a 304 Not Modified response seems to issue this correctly but I have hard times to port that code to PHP 5.

Thank you!



I've always used:

function caching_headers ($file, $timestamp) {
    $gmt_mtime = gmdate('r', $timestamp);
    header('ETag: "'.md5($timestamp.$file).'"');
    header('Last-Modified: '.$gmt_mtime);
    header('Cache-Control: public');

    if(isset($_SERVER['HTTP_IF_MODIFIED_SINCE']) || isset($_SERVER['HTTP_IF_NONE_MATCH'])) {
        if ($_SERVER['HTTP_IF_MODIFIED_SINCE'] == $gmt_mtime || str_replace('"', '', stripslashes($_SERVER['HTTP_IF_NONE_MATCH'])) == md5($timestamp.$file)) {
            header('HTTP/1.1 304 Not Modified');

Don't remember whether I wrote it or got it from somewhere else...

I'm normally using it at the top of a file in this way:

caching_headers ($_SERVER['SCRIPT_FILENAME'], filemtime($_SERVER['SCRIPT_FILENAME']));
Sunday, August 28, 2022

In your case, && and and do the same thing, but take a look at the operator precedence. You can see that && and and are not on the same level, so mixing that could give you unexpected results in some cases - I recommend always using &&, but that's your choice.

Friday, December 16, 2022

Never output anything before sending the HEADER, if you do so you will not be able to send the header and it will throw an error !
It's also a good practice to set error_reporting(0) on production server to make sure that no error gets shown before header

Friday, August 5, 2022

The solution is to ensure that you are using the mysqlnd driver for php.

How do you know that you're not using mysqlnd?

When viewing php -i, there will be no mention of "mysqlnd". The pdo_mysql section will have something like this:


PDO Driver for MySQL => enabled Client API version => 5.1.72

How do you install it?

Most installation guides for L/A/M/P suggest apt-get install php5-mysql but the native driver for MySQL is installed by a different package: php5-mysqlnd. I found that this was available with the ppa:ondrej/php5-oldstable.

To switch to the new driver (on Ubuntu):

  • Remove the old driver:
    apt-get remove php5-mysql
  • Install the new driver:
    apt-get install php5-mysqlnd
  • Restart apache2:
    service apache2 restart

How do I check that the driver is being used?

Now php -i will mention "mysqlnd" explicitly in the pdo_mysql section:


PDO Driver for MySQL => enabled
Client API version => mysqlnd 5.0.10 - 20111026 - $Id:      e707c415db32080b3752b232487a435ee0372157 $

PDO settings

Ensure that PDO::ATTR_EMULATE_PREPARES is false (check your defaults or set it):
$pdo->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);

Ensure that PDO::ATTR_STRINGIFY_FETCHES is false (check your defaults or set it):
$pdo->setAttribute(PDO::ATTR_STRINGIFY_FETCHES, false);

Returned values

  • Floating-point types (FLOAT, DOUBLE) are returned as PHP floats.
  • Integer types (INTEGER, INT, SMALLINT, TINYINT, MEDIUMINT, BIGINT †) are returned as PHP integers.
  • Fixed-Point types (DECIMAL, NUMERIC) are returned as strings.

† BIGINTs with a value greater than a 64 bit signed int (9223372036854775807) will return as a string (or 32 bits on a 32 bit system)

      public 'integer_col' => int 1
      public 'double_col' => float 1.55
      public 'float_col' => float 1.5
      public 'decimal_col' => string '1.20' (length=4)
      public 'bigint_col' => string '18446744073709551615' (length=20)
Thursday, September 22, 2022
  • It's not quite correct. Please take a look at the algorithm: alt text
  • The solution is proxy-friendly, you may use Cache-control: proxy-revalidate to force caches to obey any freshness information you give them about a resource (only applies to shared|proxy caches)

Here is the function that might help:

function isModified($mtime, $etag) {
    return !( (
        strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']) >= $mtime
    ) || (
        $_SERVER['HTTP_IF_NONE_MATCH'] == $etag
    ) ) ;

I suggest that you take a look at the following article:


[AlexV] Is is even possible to receive if-none-match AND if-modified-since at the same time?

You can definitely have both set. However:

If none of the entity tags match, then the server MAY perform the requested method as if the If-None-Match header field did not exist, but MUST also ignore any If-Modified-Since header field(s) in the request. That is, if no entity tags match, then the server MUST NOT return a 304 (Not Modified) response.

RFC2616 #14.26

Example values (W stands for 'weak'; read more in RFC2616 #13.3.3):

If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
If-None-Match: *

As a special case, the value "*" matches any current entity of the resource.

Friday, December 23, 2022
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