In a situation like this you need to check that all the variables you are using in the calculation are valid before you panic.
So try
echo 'before I use these values they contain<br>';
echo '$masonry_item_width = ' . $masonry_item_width . '<br>';
echo '$masonry_item_padding = ' . $masonry_item_padding . '<br>';
$skizzar_masonry_item_width = $masonry_item_width;
$skizzar_masonry_item_padding = $masonry_item_padding;
$skizzar_double_width_size = $masonry_item_width*2 +$masonry_item_padding;
echo 'after moving the fields to an unnecessary intemediary field<br>';
echo '$skizzar_masonry_item_width = ' . $skizzar_masonry_item_width . '<br>';
echo '$skizzar_masonry_item_padding = ' . $skizzar_masonry_item_padding . '<br>';
echo '$skizzar_double_width_size = ' . $skizzar_double_width_size . '<br>';
$output .= '<style>.skizzar_masonry_entry.skizzar_ma_double, .skizzar_masonry_entry.skizzar_ma_double img {width:'.$skizzar_double_width_size.'}</style>';
echo $output;
This should identify which fields are causing you problems.
Also while testing always run with display_errors = On It saves so much time in the long run.
$outputvariable with$output = "";before you appended stuff?$skizzar_double_width_sizehas a value?var_dump($skizzar_double_width_size)will tell you if there's anything in that var. And given your previous question, you really should sit down and study php syntax a bit better. "echo a value in an output variable"?width:485in css is illegal. it should bewidth:485pxor whatever.