Last-Modified

PHP
1

I used get_headers to get Last-Modified of a url. But returned array does not have Last-Modified key. Is there a trick I can get last-modified date of a url?

2

Are you sure the server provides that information? I suspect that if the server isn’t sending it, there’s not a lot you can do about it.

3

Yes, going by the RFC it looks to be it’s a “usually”

https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html

14.29 Last-Modified
…
HTTP/1.1 servers SHOULD send Last-Modified whenever feasible.

4

Last-Modified is not always present. Browsers send Last-Modified header only when you have set it before for the given URL. If you haven’t sent it before in your earlier response then the header is simply not there.

5

Do you mean I have to set last-modified manually as header of the page that I can get it with get_headers from another url?

6

Actually I want to detect if any change is done in text of a page. As last-modified is not present I think of this trick: I do get_file_contents then strip_tags to get text only then md5 it. Whenever md5 is different this means a change is done on page. Any better way to detect it?

7

Manually or not manually you have to send it first - but not from another url but from the same url. Actually, I was wrong in my previous answer - you can’t get the Last-Modified header at all because Last-Modified is a response header, not a request header! Browsers never send it and therefore you can’t read it with get_headers.

But the browsers will send the If-Modified-Since header after you send the Last-Modified header in your response earlier.

Suppose you have a pdf file that you want to send but only when it’s not in the browser’s cache. The scenario is like this:

  1. A user (via a browser) makes a request for http://example.com/mydoc.pdf. Because it’s the first request for this document the browser doesn’t send If-Modified-Since and you don’t receive it in get_headers. Therefore, you must send the whole file in your response and in this response you also send the Last-Modified header.

  2. A few minutes later the users makes another request for http://example.com/mydoc.pdf. This time the browser sends If-Modified-Since with the time from your previous Last-Modified header. You read it with get_headers, compare the time with the actual modification time and if it turns out the document has changed since that time then you go back to point 1 and send it again in whole. If it hasn’t changed you only send a blank 304 response and the browser will load the document from its cache.

8

What about my latest reply above?

9

You have two options:

  1. If you keep the last modified time of a page somewhere (e.g. in a database) then you can use it with Last-Modified header.

  2. You can use md5 in the etag header - it’s used analogously to Last-Modified. This is also a good solution.

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10

That given url is not mine so I cannot customize its etag or last-modified header. And that way is my trick to have a change detection for it. Any better way than what I said assuming we cannot customize any header of it?

11

track down what exactly you see as a change. if the website contains any dynamic data, like a clock, or a click-counter, you will always detect a “change”.

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12

And even if the server delivers the “last-modified” header, it’s worth checking whether it decides to always output “now” if the page contains any dynamic data. I once spent some time writing code to connect to a web page and check the last-modified date, and if it was later than my locally-stored date I’d download and index the page. Turned out, though, that because the page showed current date and time on the top, the server would notice this as a dynamic element and set last-modified to that date/time. Hence every time I checked, it looked as if it had changed.

So, analyse the content you’re interested in rather than assuming the headers tell you what you need.

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13

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