why there is a difference between below given output?

$y = 07;
echo 'Y: '.$y; // result is 7
$y = 08;
echo 'Y: '.$y; // result is 0

view demo

:EDIT:

One more similar to that

$y = 013;
echo $y + 5; //this result in 16

I can not figure it out how its ans is 16? Can any one help?

Part 1

The rules for parsing are explained in the Integers Documentation.

In PHP a number starting with a 0 is a assumed to be in Octal. Because 08 in Octal isn't valid you are getting 0.

Part 2

The same issue is in play, 013 in Octal is 11 in Decimal and 11 + 5 = 16

In php integer variable is taken Octal.

http://www.ascii.cl/conversion.htm

show This link useful for you.i think.