Question

I am calculating the size of a file upload in PHP, to format in MB with one decimal place like so:

$metadata['upload_data'] = intval($_FILES['Filedata']['size'] / 104857.6) / 10;

When I echo $metadata['upload_data'] the output is 1.7 as I would expect. But when I serialize the array with serialize($metadata) and save it to a file, the output is:

a:2:{s:7:"uploads";i:11;s:11:"upload_data";d:1.6999999999999999555910790149937383830547332763671875;}

I’m trying to be efficient by storing file sizes in MB not bytes, but this seems worse! Why would PHP store it that way? And am I going about this the right way? Thanks

Answer 1

From the manual: http://php.net/manual/en/language.types.float.php

Additionally, rational numbers that
are exactly representable as floating
point numbers in base 10, like 0.1 or
0.7, do not have an exact representation as floating point
numbers in base 2, which is used
internally, no matter the size of the
mantissa. Hence, they cannot be
converted into their internal binary
counterparts without a small loss of
precision. This can lead to confusing
results: for example,
floor((0.1+0.7)*10) will usually
return 7 instead of the expected 8,
since the internal representation will
be something like
7.9999999999999991118….

I would suggest using json_encode and json_decode if you want to see “1.7” in a serialized version of your array. These functions also end up being quicker than serialize and unserialize and are also easier to read (by easier to read, I mean a person reading them, rather than the machine).