gistfile1.php

<?php

if (isset($_POST['postcode'])) {
    $regions = array(
        'm' => 'manchester',
        'n' => 'london',
        's' => 'london',
        'b' => 'birmingham',
        'br' => 'kent',
        'ct' => 'kent',
        'da' => 'kent',
        'me' => 'kent',
        'tn' => 'kent',
        'cr' => 'surrey',
        'gu' => 'surrey',
        'kt' => 'surrey',
        'rh' => 'surrey',
        'sm' => 'surrey',
        'tn' => 'surrey',
        'tw' => 'surrey'    
    );
    $postCode = strtolower($_POST['postcode']);
    
    if (array_key_exists($postCode, $regions)) {
        $res = $regions[$postCode];

    } else if(array_key_exists(substr($postCode, 1, 1), $regions)) {
        $res = $regions[substr($postCode, 1, 1)];

    } else {
        $res = null;
    }

    if (!is_null($res)) {
      header('Location: /sell-house/' . $res . '.html');
    }
}