<?php

function rateOffset($now, $cutoff){
    if ($now < strtotime($cutoff)){
        return 20.00;
    } else {
        return 6.00;
    }
}

$cutoff = "18:56";

for ($i = 0; $i < 24; $i++) {
    $now = strtotime("$i:00");
    $rateCalc = rateOffset($now, $cutoff);
    echo date('H:i', $now)." = $rateCalc\n";
}

?>