Parsing XML document with PHP using 'foreach' loop

You write that you've got problems to debug your issues traversing an XML document with SimpleXML.

The first puzzle you come over is that your foreach does only iterate once:

foreach ($xml->alliances as $alliances) {

You can't accept the fact. However, if we take the XML you've got in your question and actually take a look how many <alliances> elements the XML document has, we can see that SimpleXML is doing the right thing here:

• there is exactly one (1) <alliances> element inside the document element.
• $xml->alliances has one (1) iteration.
• $xml->alliances->count() gives int(1)

The accordance with the XML can be easily verified as well. Commented dead code in your questions example suggests that you were using var_dump to see whether or not the XML loads. You don't have to, if simplexml_load_file does not return false, the document was loaded (if you opt for falsy: the document was either not loaded or empty).

So if you want to ensure the document has loaded, just check the return value and throw an exception in case there was a problem.

To check which XML a SimpleXMLElement contains, you shouldn't use var_dump as well. Instead output the XML. As the XML can be quite large at this point, take only the first 256 bytes for example, that normally shows a good picture:

echo substr($xml->alliances->asXML(), 0, 256), "\n";

<alliances>
    <alliance>
    <alliance id="1">Harmless?</alliance>  
    <foundedbyplayerid id="10"/><alliancecapitaltownid id="14646"/>
    <allianceticker>H?</allianceticker>  
    <foundeddatetime>2010-02-25T14:18:07.867</foundeddatetime>  
    <alliancecapitallastmoved>2012-01-19T17:42
 ^^^^^^^^^

This directly shows that you're iterating over the element(s) named alliances which exist only once in the document. This is totally aligned with the observation you've made that there is only one foreach.

With this really basic debugging you can do the following conclusion:

• It is observed that Foreach does only iterate once (1).
• Foreach has been commanded to iterates over elements named alliances.
• As there is only one (1) iteration, there has to be only one (1) alliances element.
• Counting the alliances elements, the result is one.
• Therefore it is confirmed that there is only one (1) alliances element.

So obviously you're iterating over the wrong element(s).

As this outline of the error finding is rather extensive (just to give you the picture at which many points you could have already improved both your code but also the error checking and especially to show you places where you can start with trouble-shooting), the question remains, why you weren't able to spot this already. As until now, an answer here already pointed to the fact, that you were iterating over the wrong element(s). However it was not written out, but just a bit cryptic in code:

[...] change your for loop from foreach ($xml->alliances->alliance as $alliance) { to foreach ($xml->alliance as $alliance) {

and that's all

Source

Sure it's weak, as this only gives code but doesn't answer any of your (programming) question(s).

After finding the cause, let's cure this step by step

So after finding out that it's the wrong element, it's easy to fix that: iterate over the right elements.

This can be done by applying incremental changes to your code.

First of all the correct element needs to be chosen:

foreach ($xml->alliances->alliance as $alliances) {

This will immediately make your code spit out a lot of errors, many for each iteration. And there are many iterations. So you can already say with this little change, something was effectively changed into the right direction: Instead of one iteration, there are now many more.

But before fixing the mess with the newly introduced errors and warnings, first take care about the code just changed. The next thing is to rename the variable $alliances to $alliance (your editor should support your with that by either using search and replace (often CTRL+R) or by offering a refactoring command named "rename variable" (e.g. SHIFT+F6 in Phpstorm)). Afterwards that line (and the following lines are also changed but I don't show them) looks like:

 foreach ($xml->alliances->alliance as $alliance) {

And it's yet still not ready. As $xml->alliances->alliance is a bit bulky, let's move it out and take a more speaking variable for that: $alliances:

$alliances = $xml->alliances->alliance;
foreach ($alliances as $alliance) {

The next step that needs to be done is just to correct an error you made. For some obscure reason totally not clear to me is that pass all data through mysqli_real_escape_string(). Even though if you would have intended to pass the data later on to a database, this is yet at the wrong place to call that function. First of all extract the data, that function is called later on in preparation of the database insert operation which is a different part of your application.

I just replaced all occurences of "mysqli_real_escape_string($dbconnect," with "trim(" so that finally - after proper indentation - the code has changed to this:

$alliances = $xml->alliances->alliance;
foreach ($alliances as $alliance) {

    // Alliance info
    $alliance_id   = trim($alliance->alliance->alliance['id']);
    $alliance_name = trim($alliance->alliance->alliance);

    // Diplomacy info
    $proposed_by_alliance_id = trim($alliance->alliance->relationships->relationship->proposedbyalliance['id']);
    $accepted_by_alliance_id = trim($alliance->alliance->relationships->relationship->acceptedbyalliance['id']);
    $relationship_type_id    = trim($alliance->alliance->relationships->relationship->relationshiptype['id']);
    $established_date        = trim($alliance->alliance->relationships->relationship->establishedsince);

Thanks to the better named variables it now is pretty visible where the many

Notice: Trying to get property of non-object

warnings come from: The many calls to $alliance->alliance-> are just redundant. If we remember that originally you did iterate over the wrong elements, this is the counter-part: Because you used the wrong elements, you had to make the error more than once, otherwise you could not have extracted any data at all. Just think a second about this. It also means, that the earlier you could have verified that what your intention to do is actually done by the code, the less little problems were introduced.

Good thing here again is that this is easy to fix by replacing all "$alliance->alliance->" with "$alliance->":

$alliances = $xml->alliances->alliance;
foreach ($alliances as $alliance) {

    // Alliance info
    $alliance_id   = trim($alliance->alliance['id']);
    $alliance_name = trim($alliance->alliance);

    // Diplomacy info
    $proposed_by_alliance_id = trim($alliance->relationships->relationship->proposedbyalliance['id']);
    $accepted_by_alliance_id = trim($alliance->relationships->relationship->acceptedbyalliance['id']);
    $relationship_type_id    = trim($alliance->relationships->relationship->relationshiptype['id']);
    $established_date        = trim($alliance->relationships->relationship->establishedsince);

Running the code again now shows that the iteration works and the information to obtain from each alliance element works perfectly fine as well. Still there are errors given because as you already say in your question, you not only wonder about the iteration but also about further traversing the relationships:

Alliance ID ......: 1
Alliance NAME ....: Harmless?
Diplomacy Proposed: 454
Diplomacy Accepted: 1
Diplomacy Type ...: 4
Date Accepted  ...: 2011-10-24T05:08:35.830
-------------------------------------------------
[4x Notice: Trying to get property of non-object]
Alliance ID ......: 2
Alliance NAME ....: Danger
Diplomacy Proposed: 
Diplomacy Accepted: 
Diplomacy Type ...: 
Date Accepted  ...: 
-------------------------------------------------
...

The error messages correspond to the following four lines:

$proposed_by_alliance_id = trim($alliance->relationships->relationship->proposedbyalliance['id']);
$accepted_by_alliance_id = trim($alliance->relationships->relationship->acceptedbyalliance['id']);
$relationship_type_id    = trim($alliance->relationships->relationship->relationshiptype['id']);
$established_date        = trim($alliance->relationships->relationship->establishedsince);

Which means, that again, you need to apply trouble-shooting steps as outlined at the very beginning of my answer to this section now of your code.

Here is the code example so far:

$xml = simplexml_load_file($alliances_xml); // $alliances_xml = path to file
if (!$xml) {
    throw new UnexpectedValueException(
        sprintf("Unable to load XML or it was empty. Filename given was %s", var_export($alliances_xml, true))
    );
}

$alliances = $xml->alliances->alliance;
// limit to two iterations for debugging
$alliances = new LimitIterator(new IteratorIterator($alliances), 0, 2);

foreach ($alliances as $alliance) {

    // Alliance info
    $alliance_id   = trim($alliance->alliance['id']);
    $alliance_name = trim($alliance->alliance);

    // Diplomacy info

    $proposed_by_alliance_id = trim($alliance->relationships->relationship->proposedbyalliance['id']);
    $accepted_by_alliance_id = trim($alliance->relationships->relationship->acceptedbyalliance['id']);
    $relationship_type_id    = trim($alliance->relationships->relationship->relationshiptype['id']);
    $established_date        = trim($alliance->relationships->relationship->establishedsince);

    // this is my attempt to echo every result
    echo "Alliance ID ......: $alliance_id\n";
    echo "Alliance NAME ....: $alliance_name\n";
    echo "Diplomacy Proposed: $proposed_by_alliance_id\n";
    echo "Diplomacy Accepted: $accepted_by_alliance_id\n";
    echo "Diplomacy Type ...: $relationship_type_id\n";
    echo "Date Accepted  ...: $established_date\n";
    echo "-------------------------------------------------\n";
}

Please note that I'm using the command-line to execute the PHP code as it's much faster then via the browser over a webserver. I also do not need to write HTML to just have nicely formatted output.