It concerns variables passed by reference... and the unset() function.

# Let's define a very simple function that converts an array to a string :
function foo(&$a)
{
   $an_array = array("abcdef");
   unset($a);
   
   for ($i=0; $i<sizeof($an_array); $i++)
	$a = $a . $an_array[$i];
}

# and call it :
foo($my_var);

# $my_var is NOT EQUAL TO "abcdef" !!!!!

However it works fine when we don't try to unset the $a variable. (replacing the line with $a = "").