I assume your commands above to set the variables are in a shell script. Are you calling the PHP code from the same script after setting these?
If so, you need to add this to the script after setting LOG:
Then you can try this in your PHP code:
$LogFile = getenv('LOG');
If not, the variables you have set are probably not available to the PHP code, so you have to set them again in your PHP code, as follows:
$DIR = date("mdY");
$DES = '/vol-112124';
$MYDIR = $DES . '/' . $DIR;
$Logfile = $MYDIR . '/' . date("Y-m-d") . '.log';
Hope this helps!