Algorítimo idiota (e caro) para cálculo de dias úteis entre duas datas.

businessDays.php

<?php
/**
 * Gets the business days between two dates.
 * @param string $startDate ISO 8601 date string for start date.
 * @param string $endDate ISO 8601 date string for end date.
 * @param array $dayOffs List with day offs dates in ISO 8601.
 * @return array The business days between two dates.
 */
function businessDays($startDate, $endDate, array $dayOffs = array()) {
    $startTime = strtotime($startDate);
    $endTime = strtotime($endDate);
    $businessDays = array();

    for ($currentTime = $startTime; $currentTime <= $endTime; $currentTime += 86400) {
        $dayOfWeek = date('N', $currentTime);

        if ($dayOfWeek < 6) {
            $currentDay = date('Y-m-d', $currentTime);

            $businessDays[] = $currentDay;
        }
    }

    return array_diff($businessDays, $dayOffs);
}

BusinessDaysTest.php

<?php
require_once 'PHPUnit/Framework/TestCase.php';
require_once 'businessDays.php';

/**
 * businessDays() test case.
 */
class BusinessDaysTest extends PHPUnit_Framework_TestCase {
    protected function setUp() {
        parent::setUp();

        date_default_timezone_set('America/Sao_Paulo');
    }

    public function dayOffProvider() {
        return array(array(
            array('2012-01-01',
                  '2012-04-06',
                  '2012-04-08',
                  '2012-04-21',
                  '2012-05-01',
                  '2012-09-07',
                  '2012-10-12',
                  '2012-11-02',
                  '2012-11-15',
                  '2012-12-25')
        ));
    }

    public function testFullWeek() {
        $this->assertCount(5, businessDays('2012-08-26', '2012-09-01'));
    }

    public function testWeekend() {
        $this->assertEmpty(businessDays('2012-09-01', '2012-09-02'));
    }

    /**
     * @dataProvider dayOffProvider
     */
    public function testDayOffNotIncluded(array $dayOffs) {
        $this->assertNotContains('2012-09-07', businessDays('2012-09-02', '2012-09-08', $dayOffs));
    }

    /**
     * @dataProvider dayOffProvider
     */
    public function testWeekWithDayOff(array $dayOffs) {
        $this->assertCount(4, businessDays('2012-09-02', '2012-09-08', $dayOffs));
    }
}