Question

Imagine if there are two lists and the two lists have some similar items.
i want a third list where any items shared between the first two are removed

so list1.txt:

apple

pear

bus

list2.txt:

apple

pear

output:

bus

Answer 1

You could do it like this:

// read both files into an array (each line is an element)
$list1 = file("list1.txt", FILE_IGNORE_NEW_LINES);
$list2 = file("list2.txt", FILE_IGNORE_NEW_LINES);

$merge = array_merge($list1, $list2);
$intersect = array_intersect($list1, $list2);
$not_shared = array_values(array_diff($merge, $intersect));

Lets say we have the following files:
list1.txt

apple
pear
bus

list2.txt

pear
apple
banana

Then the output would look like this: (print_r($not_shared))

Array
(
    [0] => bus
    [1] => banana
)