$fruit1 = ['apples', 'pears'];
$fruit2 = ['bananas', 'oranges'];

$all_of_fruits = array_merge($fruit1, $fruit2);
// now value of $all_of_fruits is [0 => 'apples', 1 => 'pears', 2 => 'bananas', 3 => 'oranges']

Note that array_merge will change numeric indexes, but overwrite string indexes

$fruit1 = ['one' => 'apples',  'two' => 'pears'];
$fruit2 = ['one' => 'bananas', 'two' => 'oranges'];

$all_of_fruits = array_merge($fruit1, $fruit2);
// now value of $all_of_fruits is ['one' => 'bananas', 'two' => 'oranges']

array_merge overwrites the values of the first array with the values of the second array, if it cannot renumber the index.

You can use the + operator to merge two arrays in a way that the values of the first array never get overwritten, but it does not renumber numeric indexes, so you lose values of arrays that have an index that is also used in the first array.

$fruit1 = ['one' => 'apples',  'two' => 'pears'];
$fruit2 = ['one' => 'bananas', 'two' => 'oranges'];

$all_of_fruits = $fruit1 + $fruit2;
// now value of $all_of_fruits is ['one' => 'apples', 'two' => 'pears']

$fruit1 = ['apples', 'pears'];
$fruit2 = ['bananas', 'oranges'];

$all_of_fruits = $fruit1 + $fruit2;
// now value of $all_of_fruits is [0 => 'apples', 1 => 'pears']

The array_intersect function will return an array of values that exist in all arrays that were passed to this function.

$array_one = ['one', 'two', 'three'];
$array_two = ['two', 'three', 'four'];
$array_three = ['two', 'three'];

$intersect = array_intersect($array_one, $array_two, $array_three);
// $intersect contains ['two', 'three']

Array keys are preserved. Indexes from the original arrays are not.

array_intersect only check the values of the arrays. array_intersect_assoc function will return intersection of arrays with keys.

$array_one = [1 => 'one',2 => 'two',3 => 'three'];
$array_two = [1 => 'one', 2 => 'two', 3 => 'two', 4 => 'three'];
$array_three = [1 => 'one', 2 => 'two'];

$intersect = array_intersect_assoc($array_one, $array_two, $array_three);
// $intersect contains [1 =>'one',2 => 'two']

array_intersect_key function only check the intersection of keys. It will returns keys exist in all arrays.

$array_one = [1 => 'one',2 => 'two',3 => 'three'];
$array_two = [1 => 'one', 2 => 'two', 3 => 'four'];
$array_three = [1 => 'one', 3 => 'five'];

$intersect = array_intersect_key($array_one, $array_two, $array_three);
// $intersect contains [1 =>'one',3 => 'three']

The following example shows how to merge two arrays into one associative array, where the key values will be the items of the first array, and the values will be from the second:

$array_one = ['key1', 'key2', 'key3'];
$array_two = ['value1', 'value2', 'value3'];

$array_three = array_combine($array_one, $array_two);
var_export($array_three);

/* 
    array (
      'key1' => 'value1',
      'key2' => 'value2',
      'key3' => 'value3',
    )
*/

If you have a multidimensional array like this:

[
    ['foo',  'bar'],
    ['fizz', 'buzz'],
]

And you want to change it to an associative array like this:

[
    'foo'  => 'bar',
    'fizz' => 'buzz',
]

You can use this code:

$multidimensionalArray = [
    ['foo',  'bar'],
    ['fizz', 'buzz'],
];
$associativeArrayKeys   = array_column($multidimensionalArray, 0);
$associativeArrayValues = array_column($multidimensionalArray, 1);
$associativeArray       = array_combine($associativeArrayKeys, $associativeArrayValues);

Or, you can skip setting $associativeArrayKeys and $associativeArrayValues and use this simple one liner:

$associativeArray = array_combine(array_column($multidimensionalArray, 0), array_column($multidimensionalArray, 1));