SimpleXML检查存在SimpleXML对象

Question

I am using an if statement to check whether an XML object exists, but it does not work well as I get trying to get property of non-object error

below is the code

foreach($resp->GetLowestOfferListingsForASINResult as $product){
   $isbn = "";
   $price = "";
   $condition = "";

   if($product->Product->Identifiers->MarketplaceASIN->ASIN){
      $isbn = (string) $product->Product->Identifiers->MarketplaceASIN->ASIN;
   }

   if($product->Product->LowestOfferListings->LowestOfferListing->Price->LandedPrice->Amount){
      $price = (float) $product->Product->LowestOfferListings->LowestOfferListing->Price->LandedPrice->Amount;
   }

   if($product->Product->LowestOfferListings->LowestOfferListing->Qualifiers->ItemSubcondition){
      $condition = (string) $product->Product->LowestOfferListings->LowestOfferListing->Qualifiers->ItemSubcondition;
   }
}

I have tried using

if(!property_exists($product, 'Amount')){
   $price = (float) $product->Product->LowestOfferListings->LowestOfferListing->Price->LandedPrice->Amount;
}

but that does not work, what should I be using

xml returned result (if price exists)

<?xml version="1.0"?>
<GetLowestOfferListingsForASINResponse xmlns="http://mws.amazonservices.com/schema/Products/2011-10-01">
<GetLowestOfferListingsForASINResult ASIN="000726965X" status="Success">
  <AllOfferListingsConsidered>true</AllOfferListingsConsidered>
  <Product xmlns="http://mws.amazonservices.com/schema/Products/2011-10-01" xmlns:ns2="http://mws.amazonservices.com/schema/Products/2011-10-01/default.xsd">
    <Identifiers>
      <MarketplaceASIN>
        <MarketplaceId>A1F83G8C2ARO7P</MarketplaceId>
        <ASIN>000726965X</ASIN>
      </MarketplaceASIN>
    </Identifiers>
    <LowestOfferListings>
      <LowestOfferListing>
        <Qualifiers>
          <ItemCondition>Used</ItemCondition>
          <ItemSubcondition>Mint</ItemSubcondition>
          <FulfillmentChannel>Amazon</FulfillmentChannel>
          <ShipsDomestically>True</ShipsDomestically>
          <ShippingTime>
            <Max>0-2 days</Max>
          </ShippingTime>
          <SellerPositiveFeedbackRating>95-97%</SellerPositiveFeedbackRating>
        </Qualifiers>
        <NumberOfOfferListingsConsidered>1</NumberOfOfferListingsConsidered>
        <SellerFeedbackCount>500720</SellerFeedbackCount>
        <Price>
          <LandedPrice>
            <CurrencyCode>GBP</CurrencyCode>
            <Amount>12.65</Amount>
          </LandedPrice>
          <ListingPrice>
            <CurrencyCode>GBP</CurrencyCode>
            <Amount>12.65</Amount>
          </ListingPrice>
          <Shipping>
            <CurrencyCode>GBP</CurrencyCode>
            <Amount>0.00</Amount>
          </Shipping>
        </Price>
        <MultipleOffersAtLowestPrice>False</MultipleOffersAtLowestPrice>
      </LowestOfferListing>
      <LowestOfferListing>
        <Qualifiers>
          <ItemCondition>Used</ItemCondition>
          <ItemSubcondition>Acceptable</ItemSubcondition>
          <FulfillmentChannel>Merchant</FulfillmentChannel>
          <ShipsDomestically>True</ShipsDomestically>
          <ShippingTime>
            <Max>0-2 days</Max>
          </ShippingTime>
          <SellerPositiveFeedbackRating>95-97%</SellerPositiveFeedbackRating>
        </Qualifiers>
        <NumberOfOfferListingsConsidered>1</NumberOfOfferListingsConsidered>
        <SellerFeedbackCount>73208</SellerFeedbackCount>
        <Price>
          <LandedPrice>
            <CurrencyCode>GBP</CurrencyCode>
            <Amount>15.41</Amount>
          </LandedPrice>
          <ListingPrice>
            <CurrencyCode>GBP</CurrencyCode>
            <Amount>12.61</Amount>
          </ListingPrice>
          <Shipping>
            <CurrencyCode>GBP</CurrencyCode>
            <Amount>2.80</Amount>
          </Shipping>
        </Price>
        <MultipleOffersAtLowestPrice>False</MultipleOffersAtLowestPrice>
      </LowestOfferListing>
      <LowestOfferListing>
        <Qualifiers>
          <ItemCondition>Used</ItemCondition>
          <ItemSubcondition>Mint</ItemSubcondition>
          <FulfillmentChannel>Merchant</FulfillmentChannel>
          <ShipsDomestically>True</ShipsDomestically>
          <ShippingTime>
            <Max>0-2 days</Max>
          </ShippingTime>
          <SellerPositiveFeedbackRating>98-100%</SellerPositiveFeedbackRating>
        </Qualifiers>
        <NumberOfOfferListingsConsidered>1</NumberOfOfferListingsConsidered>
        <SellerFeedbackCount>2125</SellerFeedbackCount>
        <Price>
          <LandedPrice>
            <CurrencyCode>GBP</CurrencyCode>
            <Amount>22.80</Amount>
          </LandedPrice>
          <ListingPrice>
            <CurrencyCode>GBP</CurrencyCode>
            <Amount>20.00</Amount>
          </ListingPrice>
          <Shipping>
            <CurrencyCode>GBP</CurrencyCode>
            <Amount>2.80</Amount>
          </Shipping>
        </Price>
        <MultipleOffersAtLowestPrice>False</MultipleOffersAtLowestPrice>
      </LowestOfferListing>
      <LowestOfferListing>
        <Qualifiers>
          <ItemCondition>Used</ItemCondition>
          <ItemSubcondition>Mint</ItemSubcondition>
          <FulfillmentChannel>Merchant</FulfillmentChannel>
          <ShipsDomestically>True</ShipsDomestically>
          <ShippingTime>
            <Max>0-2 days</Max>
          </ShippingTime>
          <SellerPositiveFeedbackRating>90-94%</SellerPositiveFeedbackRating>
        </Qualifiers>
        <NumberOfOfferListingsConsidered>1</NumberOfOfferListingsConsidered>
        <SellerFeedbackCount>68301</SellerFeedbackCount>
        <Price>
          <LandedPrice>
            <CurrencyCode>GBP</CurrencyCode>
            <Amount>23.14</Amount>
          </LandedPrice>
          <ListingPrice>
            <CurrencyCode>GBP</CurrencyCode>
            <Amount>20.34</Amount>
          </ListingPrice>
          <Shipping>
            <CurrencyCode>GBP</CurrencyCode>
            <Amount>2.80</Amount>
          </Shipping>
        </Price>
        <MultipleOffersAtLowestPrice>False</MultipleOffersAtLowestPrice>
      </LowestOfferListing>
      <LowestOfferListing>
        <Qualifiers>
          <ItemCondition>Used</ItemCondition>
          <ItemSubcondition>Mint</ItemSubcondition>
          <FulfillmentChannel>Merchant</FulfillmentChannel>
          <ShipsDomestically>False</ShipsDomestically>
          <ShippingTime>
            <Max>0-2 days</Max>
          </ShippingTime>
          <SellerPositiveFeedbackRating>90-94%</SellerPositiveFeedbackRating>
        </Qualifiers>
        <NumberOfOfferListingsConsidered>2</NumberOfOfferListingsConsidered>
        <SellerFeedbackCount>4114</SellerFeedbackCount>
        <Price>
          <LandedPrice>
            <CurrencyCode>GBP</CurrencyCode>
            <Amount>25.39</Amount>
          </LandedPrice>
          <ListingPrice>
            <CurrencyCode>GBP</CurrencyCode>
            <Amount>22.59</Amount>
          </ListingPrice>
          <Shipping>
            <CurrencyCode>GBP</CurrencyCode>
            <Amount>2.80</Amount>
          </Shipping>
        </Price>
        <MultipleOffersAtLowestPrice>False</MultipleOffersAtLowestPrice>
      </LowestOfferListing>
      <LowestOfferListing>
        <Qualifiers>
          <ItemCondition>Used</ItemCondition>
          <ItemSubcondition>Mint</ItemSubcondition>
          <FulfillmentChannel>Merchant</FulfillmentChannel>
          <ShipsDomestically>True</ShipsDomestically>
          <ShippingTime>
            <Max>0-2 days</Max>
          </ShippingTime>
          <SellerPositiveFeedbackRating>95-97%</SellerPositiveFeedbackRating>
        </Qualifiers>
        <NumberOfOfferListingsConsidered>1</NumberOfOfferListingsConsidered>
        <SellerFeedbackCount>34661</SellerFeedbackCount>
        <Price>
          <LandedPrice>
            <CurrencyCode>GBP</CurrencyCode>
            <Amount>29.20</Amount>
          </LandedPrice>
          <ListingPrice>
            <CurrencyCode>GBP</CurrencyCode>
            <Amount>26.40</Amount>
          </ListingPrice>
          <Shipping>
            <CurrencyCode>GBP</CurrencyCode>
            <Amount>2.80</Amount>
          </Shipping>
        </Price>
        <MultipleOffersAtLowestPrice>False</MultipleOffersAtLowestPrice>
      </LowestOfferListing>
      <LowestOfferListing>
        <Qualifiers>
          <ItemCondition>Used</ItemCondition>
          <ItemSubcondition>VeryGood</ItemSubcondition>
          <FulfillmentChannel>Merchant</FulfillmentChannel>
          <ShipsDomestically>False</ShipsDomestically>
          <ShippingTime>
            <Max>0-2 days</Max>
          </ShippingTime>
          <SellerPositiveFeedbackRating>95-97%</SellerPositiveFeedbackRating>
        </Qualifiers>
        <NumberOfOfferListingsConsidered>1</NumberOfOfferListingsConsidered>
        <SellerFeedbackCount>49504</SellerFeedbackCount>
        <Price>
          <LandedPrice>
            <CurrencyCode>GBP</CurrencyCode>
            <Amount>33.16</Amount>
          </LandedPrice>
          <ListingPrice>
            <CurrencyCode>GBP</CurrencyCode>
            <Amount>30.36</Amount>
          </ListingPrice>
          <Shipping>
            <CurrencyCode>GBP</CurrencyCode>
            <Amount>2.80</Amount>
          </Shipping>
        </Price>
        <MultipleOffersAtLowestPrice>False</MultipleOffersAtLowestPrice>
      </LowestOfferListing>
    </LowestOfferListings>
  </Product>
</GetLowestOfferListingsForASINResult>
<ResponseMetadata>
  <RequestId>09a1deae-94ec-4661-95ae-49567ab00eb8</RequestId>
</ResponseMetadata>
</GetLowestOfferListingsForASINResponse>

xml returned result (if price does not exist)

<?xml version="1.0"?>
<GetLowestOfferListingsForASINResponse xmlns="http://mws.amazonservices.com/schema/Products/2011-10-01">
<GetLowestOfferListingsForASINResult ASIN="0957143826" status="Success">
  <AllOfferListingsConsidered>true</AllOfferListingsConsidered>
  <Product xmlns="http://mws.amazonservices.com/schema/Products/2011-10-01" xmlns:ns2="http://mws.amazonservices.com/schema/Products/2011-10-01/default.xsd">
    <Identifiers>
      <MarketplaceASIN>
        <MarketplaceId>A1F83G8C2ARO7P</MarketplaceId>
        <ASIN>0957143826</ASIN>
      </MarketplaceASIN>
    </Identifiers>
    <LowestOfferListings/>
  </Product>
</GetLowestOfferListingsForASINResult>
<ResponseMetadata>
  <RequestId>13918cb9-69e5-4445-ad24-70f6fda06f19</RequestId>
</ResponseMetadata>
</GetLowestOfferListingsForASINResponse>

Answer 1

Your problem here is that you are testing too far into the chain of -> operators:

if ( $product->Product->LowestOfferListings->LowestOfferListing->Price->LandedPrice->Amount )

In order to even test this, PHP must evaluate each of the -> operators in turn

1. $product->Product [OK]
2. $product->Product->LowestOfferListings [OK]
3. $product->Product->LowestOfferListings->LowestOfferListing [An empty set if there are no prices]
4. $product->Product->LowestOfferListings->LowestOfferListing->Price [Undefined]
5. After this, you're already too late.

Additionally, the SimpleXML object won't evaluate to false even if it's an empty list, so you do need to use isset. In order to spot the no-price situation, you therefore need this:

if ( isset($product->Product->LowestOfferListings->LowestOfferListing) )

[Note: I haven't actually tested the above, so I may have gone wrong in some specific, but I'm pretty sure this is your basic problem.]