Why is the dot in a decimal replaced by comma when casted to string?

Question

In PHP I found out that when a decimal is printed to screen (or: when it's cast to string), the decimal separator will also be converted to a comma.
But why is that?
I haven't found any documentation about this, and I don't think casting to string takes internationalization settings into account.
Example code:
$var = 12.345;

var_dump($var); // Outputs: double(12.345)
var_dump((string)$var); // Outputs: string(6) "12,345"
var_dump(str_replace(',', '.', $var)); // Outputs: string(6) "12.345"
echo $var; // Outputs: 12,345

What causes this behavior?

This question applies to PHP < 8. According to the documentation,

As of PHP 8.0.0, the decimal point character is always .. Prior to PHP 8.0.0, the decimal point character is defined in the script's locale (category LC_NUMERIC). See the setlocale() function.

Bobot · Answer

If you read the manual, or check with google, you will find THIS.

The decimal point character is defined in the script's locale
  (category LC_NUMERIC)

Now, if you want to know which is the decimal point set on your sys :

print_r(localeconv());

Which returns :

Array
(
    [decimal_point] => .
    [...many other confs...]
)

If you want to set it use the good language : setlocale(LC_NUMERIC, $language)

I used fr_FR to get a comma and en_US to get a dot.

You should refer to the setlocale() manual.

By the way, I don't know how to directly set the decimal_point key of the array returned by localeconv() without changing the entire LC_NUMERIC confs, if anyone knows, please edit :)

Why is the dot in a decimal replaced by comma when casted to string?

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