Generate perfectly balanced tree

The script that follows generates perfectly balanced tree with "n" nodes:

function tree( $num_nodes )
{
 global $tree;
 
 if( $num_nodes == 0 )
  return null;
 
 $left = (int)($num_nodes / 2);
 $right = $num_nodes - $left -1;
 
 $tree[]['value'] = (int)(rand( 1, 50) / $num_nodes+1);
 $last = count($tree)-1;
 
 $tree[$last]['left'] = tree( $left );
 $tree[$last]['right'] = tree( $right );
 
 return $last;
}