$task1 = array('startDate' => '01-01-2015', 'endDate' => '10-01-2015', 'effort' => 5);
$task2 = array('startDate' => '04-01-2015', 'endDate' => '12-01-2015', 'effort' => 10);

$total = array(
    0 => array('startDate' => '01-01-2015', 'endDate' => '04-01-2015', 'effort' => 5),
    1 => array('startDate' => '04-01-2015', 'endDate' => '10-01-2015', 'effort' => 15),
    2 => array('startDate' => '10-01-2015', 'endDate' => '12-01-2015', 'effort' => 10)
);

$total = array(
    0 => array('startDate' => '01-01-2015', 'endDate' => '04-01-2015', 'effort' => 5),
    1 => array('startDate' => '04-01-2015', 'endDate' => '10-01-2015', 'effort' => 15),
    2 => array('startDate' => '10-01-2015', 'endDate' => '12-01-2015', 'effort' => 10)
);

$totalEffort = 0;
foreach ($total as $task) {
    $totalEffort += $task['effort'];
}
var_dump($totalEffort);