Methods to avoid callback hell in PHP?

For those who work with asynchronous PHP with PocketMine, how do you avoid callback hell? I currently have code with tons of nested statements. Ex:

PHP:

public function doesExist(string $blah, callable $func) : void{
    $result = lookThroughDatabase($blah);
    $func($result);
}

doesExist("hey", function(bool $bool){
    if($bool){
        keepGoingWithAsyncCallbacks();
    }
});