question about select tag in php

i have two select tags as part of a registration form, city1 city2 where
city1 has a list of regions and similar for city2

there are different regions for city1 and city2 so instead of all the
regions appearing one after the other i would like to create a blank option
followed by the next set of regions for formatting purpose only.

ex=
<select name="city1">
<option <?php if ($city1=="region1"){echo "SELECTED";}?>
value="region1">Select region1</option>
<option <?php if ($city1=="nameofregion1"){echo "SELECTED";}?>
value="nameofregion1">nameofregion1</option>
<option <?php if ($northisland=="0"){echo "SELECTED";}?> value="0"></option>
<option <?php if ($city1=="nameofregion2"){echo "SELECTED";}?>
value="nameofregion2">nameofregion2</option>
<option <?php if ($northisland=="1"){echo "SELECTED";}?> value="1"></option>
</select>


<select name="city2">
<option <?php if ($city2=="region2"){echo "SELECTED";}?>
value="region2">Select region2</option>
<option <?php if ($city2=="nameofregion1"){echo "SELECTED";}?>
value="nameofregion1">nameofregion1</option>
<option <?php if ($northisland=="2"){echo "SELECTED";}?> value="2"></option>
<option <?php if ($city2=="nameofregion2"){echo "SELECTED";}?>
value="nameofregion2">nameofregion2</option>
<option <?php if ($northisland=="3"){echo "SELECTED";}?> value="3"></option>
</select>


from a php validation perspective if a user does not select any of the
regions or both the regions i am displaying an error message asking them to
either select 1 region from either city1 or city2

as of now there is a blank option being displayed which is working fine, i
am having an issue with the php validation.

until i introduced value=0 my rules for validating the select tag were:

1. user cannot leave both the select tags with the default option which is
"Select region1" & "Select region2"
2. user cannot select both the regions from city1 & city2 select tags

the code of 2. is
if(!($city1 == "region1") && !($city2 == "region2"))
{
$error.="Please select only 1 Region<br />";
}


now by introducing <option <?php if ($northisland=="0"){echo "SELECTED";}?>
value="0"></option> there is a conflict with the above php validation code
used in point 2.

1.
is it correct to use 1,2,3 as part of the following <option> tag or should i
only use 0 everywhere
<option <?php if ($northisland=="0"){echo "SELECTED";}?> value="0"></option>
<option <?php if ($northisland=="1"){echo "SELECTED";}?> value="1"></option>

2.
how can i get around the conflict that is being created by introducing this
value=0 with
if(!($city1 == "region1") && !($city2 == "region2"))
{
$error.="Please select only 1 Region<br />";
}
as i need the above php code and i also need the blank space for formatting
purpose

please advice.

thanks.