I encountered a very strange behaviour:
The background:
Each time users uploaded images, the original file was stored along with a thumbnail and a medium sized version for small-sized slideshows.
that means that on upload all of these original images were acknowledged by php as valid jpg
The change:
I created a new slideshow script that featured full-screen display. My old "medium-sized" version were only about 600px wide which caused them to luck all ugly after stretching that to modern screen sizes.
--> I created a small script that opens all original images and saves bigger "medium-sized" versions than the once I had.
the problem:
for some reason about 1/3 of all images (not the last third but always the same images) were now labeled as "... is not a valid JPEG file ..." by php
the cause:
still unknown
the solution:
<?php
...
$src_img = @imagecreatefromjpeg($myJPGfile_relative);
if (!$src_img) {
$src_img = LoadJPEG("http://example.com/".$myJPGfile);
}
?>
Each time the local approach fails the script falls back to loading the same file "from the outside" via the below mentioned "LoadJPEG" script - big thanks to the author!
That worked even though I'm talking about the exact same file on the exact same server. The only difference is that one time it's accessed locally and the second via port 80.
when using this, watch for correct paths relative to your script and the possibly different path relative to your domain.