You can use array_uintersect in conjunction with spl_object_hash, see an example:

    array_uintersect($a, $b, function($a, $b) {
        return strcmp(spl_object_hash($a), spl_object_hash($b));
    });

If you want to get 'nature' first, because 'walk' comes before 'horse', you need to iterate over the words first, not over the tags.

$tags = [
    'animals' => ['cat', 'dog', 'horse', 'ferret'],
    'nature' => ['walk', 'outdoor', 'tree', 'plant'],
];
    
    
function getTags(string $text, array $tags): array
{
    $lowerC = strtolower($text);
    $str = preg_replace("/[^A-Za-z\'\- ]/", '', $lowerC);
    $arrayT = explode(" ", $str);

    $tagArray = [];
    foreach ($arrayT as $word) {
        // find tag for this word
        foreach ($tags as $cat => $values) {
            if (in_array($word, $values)) {
                // append the tag to the list
                $tagArray[] = $cat;
            }
        }
    }
    
    // remove duplicates
    return array_unique($tagArray);
  
}


$res = getTags('During my walk, I met a white horse', $tags);
var_dump($res);

Output :

array(2) {
  [0]=>
  string(6) "nature"
  [1]=>
  string(7) "animals"
}

EDIT

As @GeorgeGarchagudashvili mentionned, the code could be optimized by preparing an array for comparison. Here is a way :

function getTags(string $text, array $tags): array
{
    $lowerC = strtolower($text);
    $str = preg_replace("/[^A-Za-z\'\- ]/", '', $lowerC);
    $arrayT = explode(" ", $str);
    
    // Prepare tags for searching
    $searchTags = [];
    foreach ($tags as $cat => $values) {
        foreach ($values as $word) {
            $searchTags[$word] = $cat;
        }
    }

    $tagArray = [];
    foreach ($arrayT as $word) 
    {
        // find tag for this word
        if (isset($searchTags[$word])) 
        {
            // append the tag to the list
            $tagArray[] = $searchTags[$word];
        }
    }
    
    // remove duplicates
    return array_unique($tagArray);
  
}

You can do that using array_uintersect function and defining manually your callback comparison function :

$arr1 = json_decode('[{"id":2,"influencer_id":2,"follower_id":1}]');
$arr2 = json_decode('[{"id":2,"influencer_id":2,"follower_id":1},{"id":3,"influencer_id":3,"follower_id":2}]');

$arr3 = array_uintersect($arr1, $arr2, function ($e1, $e2) { 
    if($e1->id == $e2->id && $e1->influencer_id == $e2->influencer_id && $e1->follower_id == $e2->follower_id) {
        return 0;
    } else {
        return 1;
    }
});

var_dump($arr3);

Yes, it's the expected behavior, because the comparison is done using string representations, and the function does not recurse down nested arrays. From the manual:

The two values from the key => value pairs are considered equal only if

(string) $elem1 === (string)$elem2

. In other words a strict type check is executed so the string representation must be the same.

If you tried to intersect with an array with 'key1' => 'Array', you'd get the same result because the string representation of an array is always 'Array'.

One of the user-contributed notes, by nleippe, contains a recursive implementation that looks promising (I modified the third line to do string comparison on any non-array values):

function array_intersect_assoc_recursive(&$arr1, &$arr2) {
    if (!is_array($arr1) || !is_array($arr2)) {
//      return $arr1 == $arr2; // Original line
        return (string) $arr1 == (string) $arr2;
    }
    $commonkeys = array_intersect(array_keys($arr1), array_keys($arr2));
    $ret = array();
    foreach ($commonkeys as $key) {
        $ret[$key] =& array_intersect_assoc_recursive($arr1[$key], $arr2[$key]);
    }
    return $ret;
}

$ cat > test.php

<?php
$arrayOne = array('0003', '0004', '0005');
$arrayTwo = array(4, 5, 6);

$intersect = array_intersect($arrayOne, $arrayTwo);

print_r($intersect );

?>

$ php test.php

Array ( )

$

So no, it will not. But if you add

foreach($arrayOne as $key => $value)
{
   $arrayOne[$key] = intval($value);
}

you will get

$ php test.php

Array ( [1] => 4 [2] => 5 )

How about this?

foreach ($arr1 as $key => $val) {
    if (in_array($val, $arr2)){
        // do something, maybe return so you wouldn't need break
        break;
    }
}