Function always returning true no matter input - php

Question

I trying to create a function that matches the first number in a string to a state. For example if the user inputs a number that starts with 3, and the state 'vic', then the form should not present any errors. However no matter what is entered the function always returns true. Any help would be appreciated.
$state = array('Please Select', 'VIC', 'NSW', 'QLD', 'NT', 'WA', 'SA', 'TAS', 'ACT'); //In the form this is a drop down menu
$selected_key = $_POST['state'];
$postcode = $_POST["postcode"]; //The full number entered by the user
$errMsg .= validatePS($postcode, $selected_key);
function validatePS($ps, $state) {
$errMsg ="";
$digit = $ps[0]; //Takes the first number from full postcode
$valid = false;
if (($digit == 3) or ($digit == 8) && ($state == 'vic'))
{
$post = true;
}
if (($digit == 1) or ($digit == 2) && ($state == 'nsw'))
{
$post = true;
}
if (($digit == 4) or ($digit == 9) && ($state == 'qld'))
{
$post = true;
}
if ($valid == false) {
$errMsg .= "<p>Match the correct postcode to state</p>";
}
return $errMsg;
}
if ($errMsg !=""){
echo "<p>Please correct the following errors...</p>"; //Prints out errors
echo "<p>$errMsg</p>";
}

Answer 1

You have two "flag" variables - $post which you are updating and $valid which you rely on. If you reduce them to one variable, you should get the behavior you want:
function validatePS($ps, $state) {
$errMsg ="";
$digit = $ps[0]; //Takes the first number from full postcode
$valid = false;
if (($digit == 3) or ($digit == 8) && ($state == 'vic'))
{
$valid = true;
}
if (($digit == 1) or ($digit == 2) && ($state == 'nsw'))
{
$valid = true;
}
if (($digit == 4) or ($digit == 9) && ($state == 'qld'))
{
$valid = true;
}
if ($valid == false) {
$errMsg .= "<p>Match the correct postcode to state</p>";
}
return $errMsg;
}
Note that you could clean up this code considerably by using logical operators instead of multiple if statements:
function validatePS($ps, $state) {
$errMsg ="";
$digit = $ps[0]; //Takes the first number from full postcode
$valid = false;
if ((($digit == 3) or ($digit == 8) && ($state == 'vic')) ||
(($digit == 1) or ($digit == 2) && ($state == 'nsw')) ||
(($digit == 4) or ($digit == 9) && ($state == 'qld'))) {
$errMsg .= "<p>Match the correct postcode to state</p>";
}
return $errMsg;
}