SELECT option value Question

Take a look at this:
I use the $gallons variable in a select query on a MySQL
database. Values 1 thru 6 works. My question is how can I make
my query work if a user leaves in the default value of "Any".
"Any" should pull up 1 thru 6.

<SELECT name="gallons" size="1">
<option value="<? echo $gallons; ?>" selected="selected">
Any
</option>
<option value="1">
1
</option>
<option value="2">
2
</option>
<option value="3">
3
</option>
<option value="4">
4
</option>
<option value="5">
5
</option>
<option value="6">
6
</option>
</SELECT>

This is my query:
$result = mysql_query("SELECT * FROM table_name WHERE gallons='$gallons'");

I would make gallons equal 0