Clean up problems with references in php - php

Question

While studying the Garbage collector in PHP with references, I have not been understood what are the cleanup problems of references with the garbage collector that mentioned here
$a = array( 'one' );
$a[] =& $a;
xdebug_debug_zval( 'a' );
unset($a);
the reference upon this code and section said
Although there is no longer a symbol in any scope pointing to this structure, it cannot be cleaned up because the array element "1" still points to this same array. Because there is no external symbol pointing to it, there is no way for a user to clean up this structure; thus you get a memory leak.
after studying PHP references , i learned that unset the variable means cut down the binding between the variable name and content ,
so according to the below code :
$a = array( 'one' );
$a[] =& $a;
unset($a);
the whole variable $a will not be related to the content, and due to the whole array removed, then its contents references or variables are also removed, so where is the cleanup problem?
note that , according to the below code , xdebug function here generates 2 means that two references or pointers or bindings are released which proves that there isn't cleanup problem :
$a = array( 'one' );
$a[] =& $a;
xdebug_debug_zval( 'a' );
References i studied from :
Manual
Toptal Article
Sitepoint

Answer 1

The point is, this can't be removed, as there's is still a pointer to var $a. As there's no easy solution to detect, that this pointer is defined inside $a, the memory reserved for $a can't be free'd.
As seen in this image, there's a pointer from inside the array to the array itself. This pointer (which increments the refcount) exists even, if you unset the ref to $a.
$a = array( 'one' ); // refcount for a = 1
$a[] =& $a; // refcount for a = 2
unset($a); // refcount for a = 1, but there's remains no usable pointer for the php user
A simple loop could demonstrate this behavior
$start = memory_get_usage ();
for ($i = 0; $i < 100000; $i++) {
$a = ['test'];
// if you remove this line, the memory usage is 0, if not 4000000
$a[] = &$a;
unset($a);
}
echo memory_get_usage() - $start;