PHP NumberFormatter format shows strange output

Question

I have this code in a PHP function:

$fmt = NumberFormatter::create('de_DE', NumberFormatter::FRACTION_DIGITS);
echo $fmt->format(15);

output:

௰௫ 

After I did some research I found that these two characters are 15 in the Tamil language. But when I change the code like this:

$fmt = NumberFormatter::create('de_DE', NumberFormatter::FRACTION_DIGITS);
echo $fmt->format(15.5);

Then I get the expected result ie, 15.5 in German. My question is, why is Tamil coming up as a result?