How to access elements in xml when all have namespaces in PHP?

I have the following xml file. How can I reach to each element?

xml file:

<SOAP:Envelope xmlns:SOAP="http://ift.tt/sVJIaE">
    <SOAP:Body>
        <air:LowFareSearchRsp TransactionId="FC79C79F0A0764772ABBB37084848BA2"
            ResponseTime="11236" DistanceUnits="MI" CurrencyType="AUD"
            xmlns:air="http://ift.tt/1oXRNLw">
           <air:AirPricingSolution Key="0T" TotalPrice="AUD714.30"
                BasePrice="LKR51100" ApproximateTotalPrice="AUD714.30"
                ApproximateBasePrice="AUD446.00" EquivalentBasePrice="AUD446.00"
                Taxes="AUD268.30">
                <air:Journey TravelTime="P0DT11H30M0S">
                    <air:AirSegmentRef Key="109T" />
                </air:Journey>
                <air:LegRef Key="1T" />
                <air:AirPricingInfo Key="2T" TotalPrice="AUD714.30"
                    BasePrice="LKR51100" ApproximateTotalPrice="AUD714.30"
                    ApproximateBasePrice="AUD446.00" EquivalentBasePrice="AUD446.00"
                    Taxes="AUD268.30" LatestTicketingTime="2014-08-23T23:59:00.000+00:00"
                    PricingMethod="Guaranteed" Refundable="true" ETicketability="Yes"
                    PlatingCarrier="UL" ProviderCode="1G">
                    <air:FareInfoRef Key="6T" />
                    <air:BookingInfo BookingCode="W" CabinClass="Economy"
                        FareInfoRef="6T" SegmentRef="109T" />
                    <air:TaxInfo Category="EJ" Amount="AUD66.60" />
                    <air:TaxInfo Category="LK" Amount="AUD27.80" />
                    <air:TaxInfo Category="YQ" Amount="AUD173.90" />
                    <air:FareCalc>CMB UL LON 392.17WOWLK NUC392.17END ROE130.298
                    </air:FareCalc>
                    <air:PassengerType Code="ADT" />
                    <air:ChangePenalty>
                        <air:Amount>AUD43.00</air:Amount>
                    </air:ChangePenalty>
                </air:AirPricingInfo>
            </air:AirPricingSolution>
</air:LowFareSearchRsp>
    </SOAP:Body>
</SOAP:Envelope>

So far I tried with XPath. But It is not possible to write the xpath for each and every element. My php code is as below.

foreach ($xml->xpath('//SOAP:Envelope/SOAP:Body/air:AvailabilitySearchRsp/air:AirPricingSolution') as $segmentData) { 

$airSegmentRef = $segmentData->Journey->AirSegmentRef; // This is not working, shows an error. 
}

How can I access inner elements like this? Can anyone suggest how can I solve this?