My PHP nested loops use way too much memory - php

Question

I'm trying to program a utility that handles a large volume of data and memory is a factor. Unfortunately each time this set of loops I have runs, it eats apx. 14MB of memory because it is executed thousands of times, even with the unset() calls (and yes I'm aware they do not clean up memory entirely, kind of why I'm asking the question). I'm wondering if there is an easier way to do this. Current working code:
$qr = array();
foreach($XML->row as $row)
{
$ra = array();
foreach($row as $key => $value)
{
$ra[$key] = $value[0];
unset($key,$value);
}
$qr[] = $ra;
unset($row,$ra);
}
unset($XML);
return $qr;
Another attempt was to do this, but it lags out. Anybody know what I'm doing wrong?
$qr = array();
while(list(,$row) = each($XML->row))
{
$ra = array();
while(list($key,$value) = each($row))
{
$ra[$key] = $value[0];
unset($key,$value);
}
$qr[] = $ra;
unset($row,$ra);
}
unset($XML);
return $qr;
Basically in the first loop, I'm just trying to do a basic array/object iteration. In the 2nd loop, I'm trying to go through each array value and get the 1st element while maintaining object/array index association. It seems I originally wrote it like this due to it being the only thing that worked (because it's looping through a SimpleXML Object). Any tips on speeding this thing up or figuring out how to make it not eat memory would be appreciated.
I'm looking for solutions for garbage collection or more efficient code. I do not plan on replacing SimpleXML as there is no need for it. More clearly, I'm looking for:
A way to iterate the SimpleXML object without needing to call the inner loop (which is only due to me doing $value[0]. Why is that necessary?
A way which is more efficient (either speed or memory-wise) for iterating through the data

Answer 1

If you want to use less memory i recommend you start looking at SAX parser. Here is example. It is more difficult to develop parser with SAX but it's more efficient then SimpleXML, and you could parse big xml files with it.

Answer 2

Your memory load is high because SimpleXML loads the entire document into memory when parsing. So your unset() calls just decrement the reference count, and because the data still persists in memory it isn't freed. This is a consequence of working with SimpleXML: the benefit of which is that the document is in memory and represented as a PHP object.
If you want to reduce your memory usage, you need use something else like XMLReader or XML Parser. These are SAX-based, or event-based, which won't load the XML file into memory, but will walk the tree one element at a time. Since you don't appear to be using something like XPath this is your better choice.

Answer 3

That's not how you access data from a SimpleXML object. I see you are using index [0] to get the string contents of each part of the object and treating it as an array. It's not an array, it's an object. This is how you should access string data... Example: http://php.net/manual/en/simplexml.examples-basic.php#example-5095
Something like this will do the trick:
$qr = array();
foreach($XML->row as $row)
{
$ra = array();
$ra['name'] = $value->name;
$ra['name2'] = $value->name2;
//Add a line for each element name, etc...
$qr[] = $ra;
unset($row,$ra);
}
unset($XML);
return $qr;
It will also get rid of your inner loop and save you memory.