Which expert can explain the reference in this case?

For reference, who can explain the following code snippet 1: $ array_a = & nbsp; array (& nbsp; 0 & nbsp ;=& gt; & nbsp; 'A' & nbsp;); $ array_ B = & nbsp; & amp; $ array_a; $ array_c = & nbsp; who can explain this situation?
Code Segment 1:

$ Array_a = array (0 => 'A ');
$ Array_ B = & $ array_a;
$ Array_c = & $ array_ B;
$ Array_d = $ array_ B;

$ Array_c [0] = 'B ';

Echo 'variable a: '; var_dump ($ array_a );
Echo 'variable B: '; var_dump ($ array_ B );
Echo 'variable c: '; var_dump ($ array_c );
Echo 'variable d: '; var_dump ($ array_d );

Output result:

Variable:
Array (size = 1)
0 => string 'B' (length = 1)
Variable B:
Array (size = 1)
0 => string 'B' (length = 1)
Variable c:
Array (size = 1)
0 => string 'B' (length = 1)
Variable d:
Array (size = 1)
0 => string 'A' (length = 1)

The variable D value is not changed, which is easy to understand, but the reference position of variable c is changed as follows:
Code Segment 2:

$ Array_a = array (0 => 'A ');
$ Array_ B = & $ array_a;
$ Array_c = & $ array_ B [0];
$ Array_d = $ array_ B;

$ Array_c = 'B ';

Echo 'variable a: '; var_dump ($ array_a );
Echo 'variable B: '; var_dump ($ array_ B );
Echo 'variable c: '; var_dump ($ array_c );
Echo 'variable d: '; var_dump ($ array_d );

Output result:

Variable:
Array (size = 1)
0 => & string 'B' (length = 1)
Variable B:
Array (size = 1)
0 => & string 'B' (length = 1)
Variable c:
String 'B' (length = 1)
Variable d:
Array (size = 1)
0 => & string 'B' (length = 1)

Here, the variable d also changed and changed to a reference. I don't understand why.
Please advise
Reference shared :? 'A '?); $ Array_ B =? & $ Array_a; $ array_c =? & $ Array_ B; $ array_d... 'data-pics = ''>
------ Solution --------------------
As mentioned in the manual, if an array with reference is copied, its value will not be removed from the reference. This is also true for passing values from arrays to functions.

$ Array_c = & $ array_ B [0]; // so that $ array_ B has a reference, and c and B [0] point to the same memory

Example above:

 /* Common variable */
$ A = 1;
$ B = & $;
$ C = $ B;
$ C = 7; // The change of $ c does not affect $ a or $ B, and $ a and $ B still point to 1.

/* Array */
$ Arr = array (1 );
$ A = & $ arr [0]; // $ a and $ arr [0] point to the same
$ Arr2 = $ arr; // no & value assigned
$ Arr2 [0] ++;
/* $ A = 2, $ arr = array (2 )*/
/* Even if it is not a reference value, it also changes */
?>