array_intersect() / array_diff() Since: PHP 4（2000）

// Returns elements with values common to all arrays.
array_intersect(array1, array2, ...);

// Returns elements in the first array that are not in the other arrays.
array_diff(array1, array2, ...);

// Returns elements with keys common to all arrays.
array_intersect_key(array1, array2, ...);

// Returns elements whose keys are in the first array but not in the other arrays.
array_diff_key(array1, array2, ...);

<?php
$team_a = ['PHP', 'JavaScript', 'Python', 'Ruby'];
$team_b = ['JavaScript', 'Python', 'Go', 'Rust'];

// Get the skills common to both teams.
$common = array_intersect($team_a, $team_b);
print_r($common); // Returns 'JavaScript' and 'Python'.

// Get the skills in team A that are not in team B.
$only_a = array_diff($team_a, $team_b);
print_r($only_a); // Returns 'PHP' and 'Ruby'.

// Compare keys in associative arrays.
$old_settings = ['theme' => 'dark', 'lang' => 'ja', 'font_size' => 14];
$new_settings = ['theme' => 'light', 'lang' => 'en', 'sidebar' => true];

// Get elements with keys common to both arrays.
$common_keys = array_intersect_key($old_settings, $new_settings);
print_r($common_keys); // Returns the old settings values for 'theme' and 'lang'.

// Get keys in the old settings that are not in the new settings.
$removed = array_diff_key($old_settings, $new_settings);
print_r($removed); // Returns the removed setting 'font_size'.

// Get keys in the new settings that are not in the old settings.
$added = array_diff_key($new_settings, $old_settings);
print_r($added); // Returns the added setting 'sidebar'.

Both array_intersect() and array_diff() compare array values, and the keys from the first array are preserved in the returned array. Since the keys may be non-contiguous, use array_values() to reindex if needed.

Both array_intersect_key() and array_diff_key() compare by key. The values in the returned array come from the first array — values from subsequent arrays are not used. These functions are useful for detecting configuration differences and merging data.

To compare both values and keys at the same time, use array_intersect_assoc() or array_diff_assoc(). To search for a specific value within an array, in_array() / array_search() is the appropriate choice.