Help in php code

Hello all,

I a tree, and my code should move the children of one node to another one. My code doesn't give a correct result, I think I implemented it wrongly, can u help me?...

<?php
$root_node=array("parent"=>null, "child"=>"root");
$a= array("parent"=>"root", "child"=>"a");
$tree=array($root_node,$a);
$tree[]=array("parent"=>"root","child"=>"b");
?>

<?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("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_subtree(&$tree,$old,$new){
$childr[]=find_children($tree,$old);
while($childr[0]!=null){
foreach($childr as $i=>$ch){
unset($old);
$t[]=array($new,$ch);
}
}
return $t;
}
$node_x=array("root","a");

$node_c=array("root","c");

$t2[]=move_subtree($t,$node_x,$node_c);
print_r($t2);

?>