<?php


    function assignArrayByPath(&$arr, $path, $value, $separator='.') {
        $keys = explode($separator, $path);
    
        foreach ($keys as $key) {
            $arr = &$arr[$key];
        }
    
        $arr = $value;
    }

$arr = array('test'=> false);
assignArrayByPath($arr, 'test.something', true);
var_dump($arr);

array(1) {
  ["test"]=>
  array(1) {
    ["something"]=>
    bool(true)
  }
}