strlen() and calculations

[sabret00the]

i set up a while in a function to pull some details from the database, however as part of the defined function i've set up a variable in order to get the length of a string and then subtract '4' from total. however upton checking this it seems that the variable

PHP Code:

            $galleryavatarextention = strlen($galleryavatarinfo['bigimage'])-4; 


isn't being saved with or without the "-4" calculation on the end, basically it's a what am i doing wrong and am i doing this the right way type question?