<?php
$arr = array( 
    '21' => array(),
    '24' => array( 
        '22' => array(),
        '25' => array( 
            '26' => 'exists' 
        ) 
    ) 
);

var_dump(multiKeyExists($arr, 22));
var_dump(multiKeyExists($arr, 23));
var_dump(multiKeyExists($arr, 26));

function multiKeyExists(array $arr, $key) {
        
    // is in base array?
    if (array_key_exists($key, $arr)) {
        return true;
    }

    // check arrays contained in this array
    foreach ($arr as $element) {
        if (is_array($element)) {
            if (multiKeyExists($element, $key)) {
                return true;
            }
        }
        
    }

    return false;
}

bool(true)
bool(false)
bool(true)