Adds a specified number of business hours to a unix timestamp.

plus_business_hours.php

<?php
/**
 * Adds a specified number of business hours to a unix timestamp.
 *
 * @param int $time_start The timestamp at which to start
 * @param int $plus_hours The number of business hours to add
 *
 * @return int The resulting timestamp
 */
function plus_business_hours($time_start, $plus_hours)
{
    $time_end = $time_start;
    $time_info = getdate($time_end);
    $seconds = ($time_info['hours'] * 3600)
        + ($time_info['minutes'] * 60)
        + $time_info['seconds'];
    $time_end += ($time_info['hours'] < 9) ? 32400 - $seconds : 0;
    $time_end += ($time_info['hours'] >= 18) ? 118800 - $seconds : 0;
    $time_end += ($time_info['hours'] == 13) ? 50400 - $seconds : 0;
    for ($i = 0; $i < $plus_hours; ++$i) {
        do {
            $time_end += 3600;
            $time_info = getdate($time_end);
        } while ($time_info['hours'] < 9
            || $time_info['hours'] >= 18
            || $time_info['hours'] == 13
            || $time_info['wday'] == 0
            || $time_info['wday'] == 6
        );
    }
    return $time_end;
}