Using object property as default for method property

This will allow you to pass a weight of 0 and still work properly. Notice the === operator, this checks to see if weight matches "null" in both value and type (as opposed to ==, which is just value, so 0 == null == false).

PHP:

public function createShipment($startZip, $endZip, $weight=null){
    if ($weight === null)
        $weight = $this->getDefaultWeight();
}

Neat trick with Boolean OR operator:

public function createShipment($startZip, $endZip, $weight = 0){
    $weight or $weight = $this->getDefaultWeight();
    ...
}

This isn't much better:

public function createShipment($startZip, $endZip, $weight=null){
    $weight = !$weight ? $this->getDefaultWeight() : $weight;
}

// or...

public function createShipment($startZip, $endZip, $weight=null){
    if ( !$weight )
        $weight = $this->getDefaultWeight();
}