Questions about the execution process of PHP quick sort code

The question is this:
Why do two arrays, array empty and array 0 => int 6, appear when printing $left here? It stands to reason that after the recursive call reaches the array array 0 => int 1, the return $arr on line 71 is directly executed. How come there are still two arrays after that?

The source code is as follows:

<code>$arr = array(6,3,8,6,4,2,9,5,1);

function quick_sort($arr){
    if(!is_array($arr)) return false;
    
    $length = count($arr);
    if($length <= 1) return $arr;

    $left = $right = array();

    for($i = 1;$i < $length;$i++){                                                
        if($arr[$i] < $arr[0]){
            $left[] = $arr[$i];                             
        }else{
            $right[] = $arr[$i];                                       
        }
    }

    var_dump($left);
    $left = quick_sort($left);
    $right = quick_sort($right);

    return array_merge($left,array($arr[0]),$right);
}

print_r(quick_sort($arr));</code>

Reply content:

The question is this:
Why do two arrays, array empty and array 0 => int 6, appear when printing $left here? It stands to reason that after the recursive call reaches the array array 0 => int 1, the return $arr on line 71 is directly executed. How come there are still two arrays after that?

The source code is as follows:

<code>$arr = array(6,3,8,6,4,2,9,5,1);

function quick_sort($arr){
    if(!is_array($arr)) return false;
    
    $length = count($arr);
    if($length <= 1) return $arr;

    $left = $right = array();

    for($i = 1;$i < $length;$i++){                                                
        if($arr[$i] < $arr[0]){
            $left[] = $arr[$i];                             
        }else{
            $right[] = $arr[$i];                                       
        }
    }

    var_dump($left);
    $left = quick_sort($left);
    $right = quick_sort($right);

    return array_merge($left,array($arr[0]),$right);
}

print_r(quick_sort($arr));</code>

What you dump is left. If the array is [1, 3, 3], it is empty, and [2, 3, 1] is one

$arr = array(6,3,8,6,4,2,9,5,1);
$result = array();
function aaa($arr,$result){

<code>$min = min($arr);
array_push($result, $min);
$key=array_search($min ,$arr);
array_splice($arr,$key,1);
if(count($arr)){
    aaa($arr,$result);
}else{
    echo json_encode($result);
    exit;
}</code>

}
aaa($arr,$result);

It feels simpler to use this idea