Be careful when using a variable for the file name.
PHP behavior with $filename differs when switching to PHP5.4 : PHP5.3 will use $filename='' the same way as $filename=NULL (e.g. no warning)
<?php
$im = imagecreatetruecolor(10,10);
imagepng($im,'',9); # Warning: imagepng(): Filename cannot be empty
imagepng($im,NULL,9); # works as expected
imagedestroy($im);
?>