The following script

<?php
$a=3.14;
echo "<pre>$a type ". gettype($a);

$olc=setlocale(LC_ALL,0);
setlocale(LC_ALL,'de_DE');

echo "\n$a type ". gettype($a);

$ser=serialize($a);

echo "\nSER: #$ser#\n";

setlocale(LC_ALL,$olc);

$b=unserialize($ser);

echo "$b  type ". gettype($b);
?>

.... produces this output:

3.14 type double
3,14 type double
SER: #d:3,14;#
3  type double

This type of error can happen in cases, where a localized user-fronted stores some vars  for example in a Database via serialize and another program (e.g. background process, which doesn't need to be localized) reads it. In those cases, the result is not correct (=error). The problem is, that this is not obvious, because a "normal" programmer trust on serialize(), because the documentation says

     "serialize() returns a string containing a byte-stream representation of value
     that can be stored anywhere."

In other words: You can trust serialize, that it always can be unserialized.

Opportunities to solve the problem:

- serialize() stores the vars langague independed (always use '.').
- unserialize() reads double vars either with '.' or ','.
- both
- unserialize() checks, if serialized string and and resulting type is identical, if not returns with error.
- Big hint in docs

Maybe it's a good idea to check (un)serialize in general for similar errors.