upload images - preg_replace

I am using the statement below to clean up an image file name before it's uploaded. I seem to be getting an occasional server error an my images are sometimes pixelated. I don't know whether it's my code or the server that causing the problem. What's the best way to rename the uploaded file?

Renaming code at the top of the page:
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if(isset($_FILES['item_photo']['name'])) {
$_FILES['item_photo']['name'] = preg_replace('/[^A-Za-z0-9.]/','_',$_FILES['item_photo']['name']);
}

what is the error?

I dont believe that code has anything to do with the error, more likely it is a server memory issue.

Thanks Jason