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Trying to make sense of $stack[count($stack)] = &$tree;

Posted on 2008-10-20
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Last Modified: 2012-05-05
I'm trying to roll an XML file into a PHP array using Expat Parser, and I'm relatively new to PHP.  I came across the following code: http://us.php.net  /xml_set_element_handler.  (The sample code is about 1/3 the way down the page, author: "rubentrancoso at gmail dot com".)

I'm struggling with the last line of code in the following code block:

$file = "flow/flow.xml";
$depth = 0;
$tree = array();
$tree['name'] = "root";
$stack[count($stack)] = &$tree;

1) As I understand it, count returns the number of elements in the array, but there are no elements in the array, because $stack was just declared.  Right?
2) What's up with &$tree?  

(I have also attached the full sample of the code.)  
<?php
 
$file = "flow/flow.xml";
$depth = 0;
$tree = array();
$tree['name'] = "root";
$stack[count($stack)] = &$tree;
 
function startElement($parser, $name, $attrs) {
   global $depth;
   global $stack;
   global $tree;
  
   $element = array();
   $element['name'] = $name;
   foreach ($attrs as $key => $value) {
        //echo $key."=".$value;
        $element[$key]=$value;
    }
 
   $last = &$stack[count($stack)-1];
   $last[count($last)-1] = &$element;
   $stack[count($stack)] = &$element;
 
   $depth++;
}
 
function endElement($parser, $name) {
   global $depth;
   global $stack;
 
   array_pop($stack);
   $depth--;
}
 
$xml_parser = xml_parser_create();
xml_set_element_handler($xml_parser, "startElement", "endElement");
if (!($fp = fopen($file, "r"))) {
   die("could not open XML input");
}
 
while ($data = fread($fp, 4096)) {
   if (!xml_parse($xml_parser, $data, feof($fp))) {
       die(sprintf("XML error: %s at line %d",
                   xml_error_string(xml_get_error_code($xml_parser)),
                   xml_get_current_line_number($xml_parser)));
   }
}
xml_parser_free($xml_parser);
$tree = $stack[0][0];
echo "<pre>";
print_r($tree);
echo "</pre>";
?>

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Question by:jdana
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4 Comments
 
LVL 15

Expert Comment

by:MMDeveloper
ID: 22758179
stack[count($stack)] is the same as

$stack[] =

it simply says that the following value will be appended to the end of the array.


&$tree is a reference pointer per say. That means that whatever changes are made to the value in $stack[count($stack)] are replicated automatically to the original $tree value.


If I set this

$a = 5;
$b = &$a;

$b = 0;

now both $b and $a = zero. What happens to $b also happens to $a automatically.
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Author Comment

by:jdana
ID: 22758813
Does $b = &$a; set $b equal to $a BY REFERENCE as opposed to BY VALUE?
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LVL 15

Accepted Solution

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MMDeveloper earned 250 total points
ID: 22758924
you are correct.
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Author Closing Comment

by:jdana
ID: 31507826
Thanks!
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