<?php
function recursive_search($val, $arr, $mode=1, $strict=true)
{
    $depth = ($depth=(int)$mode) ? $depth : 0;
    if (strpos($mode, 'all')!==false) {
        $r = function ($val, $a, $d, $s, $l=0, &$path=[], &$keys=[], &$i=0) use (&$r) {
            foreach($a as $k=>$v) {
                $path[] = $k;
                if (!$s && $v==$val || $v===$val)  $keys[$i++] = $path;
                elseif ($l<$d && (is_array($v) /*|| method_exists($v,'getIterator')*/))
                    $r($val, $v ,$d, $s, $l, $path, $keys, $i);
                array_pop($path);
            }
            if ($l!==0) return $keys ? $keys : false;
            return array_values( $keys );
        };
    } else {
        $r = function ($val, $a, $d, $s, $l=0, $keys=array()) use (&$r) {
            foreach($a as $k=>$v) {
                if (!$s && $v==$val || $v===$val) { $keys[]=$k; return $keys; }
                elseif ($l<$d && (is_array($v) /*|| method_exists($v,'getIterator')*/)) {
                    $keys[]=$k;
                    if ($keys=$r($val, $v ,$d, $s, $l+1, $keys)) return $keys;
                }
            }
            return false;
        };
    }
    return $r($val, $arr, $depth, $strict);
}

$arr = array(
    0=>array(1=>array(2=>'depth2') ),
    '1.4' => 0,
    'depth1'=>array('depth1'),
    'arr1'=>array(
        'arr2'=>array(
            'arr3'=>array('findme'),  
        ),
    ),
    'findme1'=>array('findme'),
);
print_vals(recursive_search('depth1', $arr));          # ['depth1', 0]
print_vals(recursive_search('depth1', $arr, 0));       # [ ]
print_vals(recursive_search('depth1', $arr, 0, false));# ['1.4'] beware! non-strict
print_vals(recursive_search('depth2', $arr));    # [ ]  default depth is 1
print_vals(recursive_search('depth2', $arr, 2)); # [0,1,2]  depth now 2
print_vals(recursive_search('findme', $arr, 3)); # ['arr1','arr2','arr3',0] 

// note that the value 'findme' occured twice but we only caught one
print_vals(recursive_search('findme', $arr, '3 all')); # ['arr1','arr2','arr3',0] 
/* a third argument containing 'all' and optionally starting with an int
 * will return an array of recursive_search values: 
    [
        ['arr1', 'arr2', 'arr3', 0],
        ['findme1', 0],
    ]
 */

////////////////////////////////////////////////////////////////////////////////
// just to display demo:
function print_vals($arr) {
    if (!$arr) { echo "[ ]\n"; return; }
    foreach ($arr as $v) { if (is_array($v)) {$run_sub=true; break;} }
    if (isset($run_sub)) {
        $subs = function ($arr) {
            echo "[\n";
            foreach ($arr as $sub) {
                if (!$sub) { echo "[ ]\n"; return; }
                $s = is_int($sub[0]) ? "\t[${sub[0]}": "\t['${sub[0]}'"; unset($sub[0]);
                foreach($sub as$v)  $s = is_int($v) ? "$s, $v" : "$s, '$v'";
                echo "$s],\n";    
            }
            echo "]\n";   
        };
        return $subs($arr);
    }
    $s = is_int($arr[0]) ? "[${arr[0]}": "['${arr[0]}'"; unset($arr[0]);
    foreach($arr as$v)  $s = is_int($v) ? "$s,$v" : "$s,'$v'";
    echo "$s]\n";
}