[PHP] Tree-like structure in PHP?

[Paul van Haren]

I'm trying to use arrays to implement a sort of data tree. For the code to
work, it is essential that the nodes can be edited after they have become
part of the tree. That would require that the array elements are pushed
"by reference" rather than "by value". 

        <?php

        $arr1 = array ();
        $arr2 = array ();

        array_push ($arr1, 1);
        array_push ($arr1, $arr2);      // <-- the important line
        array_push ($arr1, 2);
        array_push ($arr2, 3);
        array_push ($arr2, 4);

        // Contents of $arr2 are now empty
        // apparently $arr2 is inserted by value
        print_r ($arr1);

        echo "Expected output:\n";
        $arr1[1] = $arr2;
        print_r ($arr1);
        ?>

What is the most obvious way to implement a tree-like structure in PHP? Is
there a way to push arrays by reference rather than by value?

BTW: I'm using PHP CLI version 5.2

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