Function instead of numbers, sometimes returns Null

Function instead of numbers, sometimes returns Null

Hi i have following code:
function GetID($d1, $d2, $d3, $d4) {
if ($d1 == '3') {
if ($d2 == '3') {
if ($d3 == 1) {
$randval
=
mt_rand(1,100);
if
($randval
>=
1
&&
$randval
<=
33)
$weathern
=
1;
if
($randval
>=
34
&&
$randval
<=
66)
$weathern
=
3;
if
($randval
>=
67
&&
$randval
>=
100)
$weathern
=
2;
}
elseif
($d3
==
2)
{
$randval
=
mt_rand(1,100);
if
($randval
>=
1
&&
$randval
<=
45)
$weathern
=
1;
if
($randval
>=
46
&&
$randval
<=
70)
$weathern
=
2;
if
($randval
>=
71
&&
$randval
<=
100)
$weathern
=
3;
}
elseif
($d3
==
3)
{
$randval
=
mt_rand(1,100);
if
($randval
>=
1
&&
$randval
<=
10)
$weathern
=
1;
if
($randval
>=
11
&&
$randval
<=
35)
$weathern
=
2;
if
($randval
>=
36
&&
$randval
<=
55)
$weathern
=
3;
if
($randval
>=
56
&&
$randval
<=
85)
$weathern
=
5;
if
($randval
>=
86
&&
$randval
<=
100)
$weathern
=
6;
}
elseif
($d3
==
5)
{
$randval
=
mt_rand(1,100);
if
($randval
>=
1
&&
$randval
<=
30)
$weathern
=
3;
if
($randval
>=
31
&&
$randval
<=
45)
$weathern
=
4;
if
($randval
>=
46
&&
$randval
<=
75)
$weathern
=
6;
if
($randval
>=
76
&&
$randval
<=
100)
$weathern
=
7;
}
elseif
($d3
==
6)
{
$randval
=
mt_rand(1,100);
if
($randval
>=
1
&&
$randval
<=
30)
$weathern
=
5;
if
($randval
>=
31
&&
$randval
<=
60)
$weathern
=
7;
if
($randval
>=
61
&&
$randval
<=
75)
$weathern
=
2;
if
($randval
>=
76
&&
$randval
<=
100)
$weathern
=
3;
}
elseif
($d3
==
7)
{
$randval
=
mt_rand(1,100);
if
($randval
>=
1
&&
$randval
<=
35)
$weathern
=
8;
if
($randval
>=
36
&&
$randval
<=
60)
$weathern
=
5;
if
($randval
>=
61
&&
$randval
<=
85)
$weathern
=
2;
if
($randval
>=
86
&&
$randval
<=
100)
$weathern
=
3;
}
elseif
($d3
==
8)
{
$randval
=
mt_rand(1,100);
if
($randval
>=
1
&&
$randval
<=
35)
$weathern
=
3;
if
($randval
>=
36
&&
$randval
<=
60)
$weathern
=
2;
if
($randval
>=
61
&&
$randval
<=
85)
$weathern
=
1;
if
($randval
>=
86
&&
$randval
<=
100)
$weathern
=
5;
}
}
}
return $weathern;
}
And when i call this function:
echo GetID(3, 3, 1, 1);
It works as expected (i get numbers) but sometimes it returns Nothing.
Any ideas why this happens?
I'm new with PHP but i don't see any problems here