Any one can explain to me? why it's returning 83?

Yes anything that starts with0 is an octal integer.

If you don't want to store string and want to store number then following are2 options.

$k =0123.00;

OR if you want to stick with integer then you can do casting.

$k = (int)0123.00;

Because it's assigning $k to the octal value of0123 which is83.

0123 =83 in octal

Instead, try to store0123 as a string e.g. $k = "0123";

This is OCTAL Number so this will be calculated as:

1*8^2+2*8^1+3*8^0 =