Don't confuse PHP & reference with other languages' reference.

tomodachi.php

<?php

function test_copy_on_write()
{
    print "test_copy_on_write\n";

    $a = 100;
    xdebug_debug_zval('a');
    //==> a: (refcount=1, is_ref=0)=100

    $b = $a;
    xdebug_debug_zval('a');
    xdebug_debug_zval('b');
    //==> a: (refcount=2, is_ref=0)=100
    //==> b: (refcount=2, is_ref=0)=100
    // ($a and $b point the same value.)

    $c = $a;
    xdebug_debug_zval('a');
    xdebug_debug_zval('b');
    xdebug_debug_zval('c');
    //==> a: (refcount=3, is_ref=0)=100
    //==> b: (refcount=3, is_ref=0)=100
    //==> c: (refcount=3, is_ref=0)=100
    // ($a, $b and $c point the same value.)

    $a++;
    xdebug_debug_zval('a');
    xdebug_debug_zval('b');
    xdebug_debug_zval('c');
    //==> a: (refcount=1, is_ref=0)=101
    //==> b: (refcount=2, is_ref=0)=100
    //==> c: (refcount=2, is_ref=0)=100
    // ($a points a different value because of copy-on-write.)
    // ($b and $c still point the same value.)

    unset($c);
    xdebug_debug_zval('a');
    xdebug_debug_zval('b');
    xdebug_debug_zval('c');
    //==> a: (refcount=1, is_ref=0)=101
    //==> b: (refcount=1, is_ref=0)=100
    //==> c: no such symbol
    // (The refcount of the value pointed by $b is decremented, because $c is unset.)
}

function test_no_copy_on_write_for_reference()
{
    print "test_no_copy_on_write_for_reference\n";

    $a = 100;
    xdebug_debug_zval('a');
    //==> a: (refcount=1, is_ref=0)=100

    $b = &$a;
    xdebug_debug_zval('a');
    xdebug_debug_zval('b');
    //==> a: (refcount=2, is_ref=1)=100
    //==> b: (refcount=2, is_ref=1)=100
    // ($a and $b point the same value.)

    $a++;
    xdebug_debug_zval('a');
    xdebug_debug_zval('b');
    //==> a: (refcount=2, is_ref=1)=101
    //==> b: (refcount=2, is_ref=1)=101
    // ($a and $b point the same value even after modification.)
}

function test_but_copy_on_assignment_from_referenct_to_value()
{
    print "test_but_copy_on_assignment_from_referenct_to_value\n";

    $a = 100;
    xdebug_debug_zval('a');
    //==> a: (refcount=1, is_ref=0)=100

    $b = &$a;
    xdebug_debug_zval('a');
    xdebug_debug_zval('b');
    //==> a: (refcount=2, is_ref=1)=100
    //==> b: (refcount=2, is_ref=1)=100
    // ($a and $b point the same value.)

    $c = $a;
    xdebug_debug_zval('a');
    xdebug_debug_zval('b');
    xdebug_debug_zval('c');
    //==> a: (refcount=2, is_ref=1)=100
    //==> b: (refcount=2, is_ref=1)=100
    //==> c: (refcount=1, is_ref=0)=100
    // ($a and $b point the same value.)
    // (But $c is a copy of that value, because changes to $a/$b must not be shared with $c.)
}

function test_reference_becomes_non_reference()
{
    print "test_reference_becomes_non_reference\n";

    $a = 100;
    xdebug_debug_zval('a');
    //==> a: (refcount=1, is_ref=0)=100

    $b = &$a;
    xdebug_debug_zval('a');
    xdebug_debug_zval('b');
    //==> a: (refcount=2, is_ref=1)=100
    //==> b: (refcount=2, is_ref=1)=100
    // ($a and $b point the same value.)

    $a++;
    xdebug_debug_zval('a');
    xdebug_debug_zval('b');
    //==> a: (refcount=2, is_ref=1)=101
    //==> b: (refcount=2, is_ref=1)=101
    // ($a and $b point the same value even after modification.)

    unset($);
    xdebug_debug_zval('a');
    xdebug_debug_zval('b');
    //==> a: (refcount=1, is_ref=0)=101
    //==> b: no such symbol
    // (Note that is_ref is changed to 0.)
    // (Because PHP's & reference is meaningful if two or more variables share the same value)
}

test_copy_on_write();
test_no_copy_on_write_for_reference();
test_but_copy_on_assignment_from_referenct_to_value();
test_reference_becomes_non_reference();