We found this method to be of value when checking most image file types. However, it does not appear to be rigorous enough to detect corrupt jpg files. We use the following for jpgs and validate() for everything else.
<?php
// check for the existence of the EOI segment header at the end of the file
if (0 !== fseek($file, -2, SEEK_END) || "\xFF\xD9" !== fread($file, 2)) {
fclose($file);
return FALSE;
}
?>