foreach($images as $key => $image)
{
    if(in_array($image, array(
       'http://i27.tinypic.com/29ykt1f.gif',
       'http://img3.abload.de/img/10nxjl0fhco.gif',
       'http://i42.tinypic.com/9pp2tx.gif',
    ))
    {
        unset($images[$key]);
    }
}

If you also get the key, you can delete that item like this:

foreach ($display_related_tags as tag_name) {
    if($tag_name == $found_tag['name']) {
        unset($display_related_tags[$key]);
    }
}

foreach($arr as $k => $v) {
    if(key($v) > 5) {
        unset($arr[$k]);
    }
}

You're unsetting the reference (breaking the reference). You'd need to unset based on a key:

foreach ($this->result['list'] as $key => &$row) {
    if ($this_row_is_boring) {
        unset($this->result['list'][$key]);
    }
}

You need to use the key from your foreach, and unset the variable directly (from the session):

foreach ($_SESSION['cart']  as $key => $arrays3) {
    if($arrays3['id'] == $id){
        unset($_SESSION['cart'][$key]);
    }
}

Unsetting $arrays3 or any of its children will only be effective until the next iteration of the foreach loop, when it will be set again.

I know this post is old, but I think this info is very important: You asked:

Is it necessary to unset($value), when $value remains after foreach loop, is it good practice?

It depends on wheter you iterate by value or by reference.

First case (by value):

$array = array(1, 2, 3, 4);
foreach ($array as $value) {

    //here $value is 1, then 2, then 3, then 4

}

//here $value will continue having a value of 4
//but if you change it, nothing happens with your array

$value = 'boom!';
//your array doesn't change.

So it's not necessary to unset $value

Second case (by reference):

$array = array(1, 2, 3, 4);
foreach ($array as &$value) {

    //here $value is not a number, but a REFERENCE.
    //that means, it will NOT be 1, then 2, then 3, then 4
    //$value will be $array[0], then $array[1], etc

}

//here $value it's not 4, it's $array[3], it will remain as a reference
//so, if you change $value, $array[3] will change

$value = 'boom!'; //this will do:  $array[3] = 'boom';
print_r ($array); //Array ( [0] => 1 [1] => 2 [2] => 3 [3] => boom! )

So in this case it is a good practice to unset $value, because by doing that, you break the reference to the array. Is it necessary? No, but if you don't do it, you should be very careful.

It can lead to unexpected results like this one:

$letters = array('a', 'b', 'c', 'd');

foreach ($letters as &$item) {}
foreach ($letters as  $item) {}

print_r ($letters); //output: Array ( [0] => a [1] => b [2] => c [3] => c )
// [a,b,c,c], not [a,b,c,d]

This is also valid for PHP 7.0

Same code with unset:

$letters = array('a', 'b', 'c', 'd');

foreach ($letters as &$item) {}
unset ($item);
foreach ($letters as  $item) {}

print_r ($letters); //output: Array ( [0] => a [1] => b [2] => c [3] => d )
// now it's [a,b,c,d]

I hope this can be useful for someone in the future. :)

In my opinion, the best way to remove array elements based on indexes is to use the array_* set of functions, like array_diff and array_intersect (or array_diff_key and array_intersect_key in your situation).

$indexes_to_remove = array(2,3,4);
$indexes_to_remove = array_flip($indexes_to_remove);

$array = array_diff_key($array,$indexes_to_remove);