Data Structure in PHP

Hello All,

I am trying to solve a question about Trees in PHP, I hope any one can help.

The question is moving children from one parent of the tree to another one. I have an error in my code telling me (Warning: Invalid argument supplied for foreach() in C:\Program Files\nusphere\phped\Projects\trial.php5 on line 18)....
I think I implemented the code wrongly, any suggestions to solve it?...I will be really thankful.


<?php
$t[]=array(null,"root");
$t[]=array("root","a");
$t[]=array("root","b");
$t[]=array("root","c");
$t[]=array("a","d");
$t[]=array("a","e");
$t[]=array("a","f");
$t[]=array("c","g");
$t[]=array("c","h");
$t[]=array("e","x");
$t[]=array("e","y");
$t[]=array("f","z");
?>

<?php
function find_children($tree,$parent){
foreach($tree as $node){
if($node[0]==$parent[1]){
$children[]=$node;
}
}
return $children;
}
//-------------------------------------
$node_a=array("root","a");
$children=find_children($t,$node_a);
?>

<?php
function move(&$tree,$old,$new){
$old=$parent;
$child[]=find_children($tree,$old);
while($child[0]!=null){
foreach($child as $i=>$ch){
$t2=array_push($new,$children);
}
}
return $t2;
}
$node_x=array("root","a");

$node_c=array("root","c");
$node_g=array("c","g");
$t=move(&$tree,$node_x,$node_c);
print_r($t);
?>