Is it true that unsetting variables doesn't actually decrease the memory consumption during runtime?

Yep. From PHP.net:

unset() does just what it's name says - unset a variable. It does not force immediate memory freeing. PHP's garbage collector will do it when it see fits - by intention as soon, as those CPU cycles aren't needed anyway, or as late as before the script would run out of memory, whatever occurs first.

If you are doing $whatever = null; then you are rewriting variable's data. You might get memory freed / shrunk faster, but it may steal CPU cycles from the code that truly needs them sooner, resulting in a longer overall execution time.

Regarding your other question:

And is there any reason to unset variables apart from destroying session varaibles for instance or for scoping?

Not really, you pretty much summed it.

Let's consider the function:

function foo() {
    static $bar;
    $bar++;
    unset($bar);
}
foo(); //static $bar is 1
foo(); //static $bar is 2

The function compiles to:

function name:  foo
number of ops:  11
compiled vars:  !0 = $bar
line     # *  op                           fetch          ext  return  operands
---------------------------------------------------------------------------------
   2     0  >   EXT_NOP                                                  
   4     1      EXT_STMT                                                 
         2      FETCH_W                      static              $0      'bar'
         3      ASSIGN_REF                                               !0, $0
   5     4      EXT_STMT                                                 
         5      POST_INC                                         ~1      !0
         6      FREE                                                     ~1
   6     7      EXT_STMT                                                 
         8      UNSET_VAR                                                !0
   7     9      EXT_STMT                                                 
        10    > RETURN                                                   null

A variable actually exists outside each function call to foo() and, on each call, it's fetched and a reference to it is assigned to $bar. In fact, it's very similar to this:

function foo() {
    global $bar;
    $bar++;
    unset($bar);
}

When you call unset(), you're only destroying the reference you created, not the underlying value.

I didn't confirm, but what I'd guess that happens is this:

• The underlying representation of the variabe (the zval) is stored so that its reference count is 1.
• When foo() is called, the symbol $bar is associated with this zval, its reference count is increased to 2 and the reference flag is set.
• When unset is called, the zval has its reference count decreased to 1, the reference flag is probably cleared and the symbol $bar is removed.

See reference count basics.

As of PHP 5.3, you can assign an anonymous function to a variable, then unset it:

Copy$upper = function($str) {
    return strtoupper($str);
};

echo $upper('test1');
// outputs: TEST1

unset($upper);

echo $upper('test2');
// Notice: Undefined variable: upper
// Fatal error: Function name must be a string

Before 5.3, you can do something similar with create_function()

Copy$func = create_function('$arg', 'return strtoupper($arg);');
echo $func('test1');
unset($func);

$func2 = "\0lambda_1";
echo $func2('test2.a'), "\n"; // Same results, this is the "unset" $func function

echo $func('test2.b'); // Fatal Error

There are different ways to delete an array element, where some are more useful for some specific tasks than others.

Deleting a Single Array Element

If you want to delete just one single array element you can use unset() and alternatively array_splice().

By key or by value?

If you know the value and don't know the key to delete the element you can use array_search() to get the key. This only works if the element doesn't occur more than once, since array_search() returns the first hit only.

unset() Expression

Note: When you use unset() the array keys won’t change. If you want to reindex the keys you can use array_values() after unset(), which will convert all keys to numerically enumerated keys starting from 0 (the array remains a list).

Example Code:

$array = [0 => "a", 1 => "b", 2 => "c"];
unset($array[1]);
          // ↑ Key of element to delete

Example Output:

[
    [0] => a
    [2] => c
]

array_splice() Function

If you use array_splice() the (integer) keys will automatically be reindex-ed, but the associative (string) keys won't change — as opposed to array_values() after unset(), which will convert all keys to numerical keys.

Note: array_splice() needs the offset, not the key, as the second parameter; offset = array_flip(array_keys(array))[key].

Example Code:

$array = [0 => "a", 1 => "b", 2 => "c"];
array_splice($array, 1, 1);
                  // ↑ Offset of element to delete

Example Output:

[
    [0] => a
    [1] => c
]

array_splice(), same as unset(), take the array by reference. You don’t assign the return values back to the array.

Deleting Multiple Array Elements

If you want to delete multiple array elements and don’t want to call unset() or array_splice() multiple times you can use the functions array_diff() or array_diff_key() depending on whether you know the values or the keys of the elements to remove from the array.

array_diff() Function

If you know the values of the array elements which you want to delete, then you can use array_diff(). As before with unset() it won’t change the keys of the array.

Example Code:

$array = [0 => "a", 1 => "b", 2 => "c", 3 => "c"];
$array = array_diff($array, ["a", "c"]);
                         // └────────┘
                         // Array values to delete

Example Output:

[
    [1] => b
]

array_diff_key() Function

If you know the keys of the elements which you want to delete, then you want to use array_diff_key(). You have to make sure you pass the keys as keys in the second parameter and not as values. Keys won’t reindex.

Example Code:

$array = [0 => "a", 1 => "b", 2 => "c"];
$array = array_diff_key($array, [0 => "xy", "2" => "xy"]);
                              // ↑           ↑
                              // Array keys of elements to delete

Example Output:

[
    [1] => b
]

If you want to use unset() or array_splice() to delete multiple elements with the same value you can use array_keys() to get all the keys for a specific value and then delete all elements.

array_filter() Function

If you want to delete all elements with a specific value in the array you can use array_filter().

Example Code:

$array = [0 => "a", 1 => "b", 2 => "c"];
$array = array_filter($array, static function ($element) {
    return $element !== "b";
    //                   ↑
    // Array value which you want to delete
});

Example Output:

[
    [0] => a
    [2] => c
]