A function that executes a list of functions, gathering necessary dependencies as it goes.

callFunctionDependencyTree.php

<?php
public static function callFunctionDependencyTree($functions) {

    do {
      $funcDidExecute = FALSE;

      $functions = array_map(function ($spec) use ($functions, &$funcDidExecute) {

        if (array_key_exists('value', $spec)) {
          return $spec;
        }

        $funcCanExecute = TRUE;
        $args = [];
        foreach ($spec['deps'] as $i => $dep) {
          if (!array_key_exists('value', $functions[$dep])) {
            $funcCanExecute = FALSE;
            break;
          }
          $args[$i] = $functions[$dep]['value'];
        }

        if ($funcCanExecute) {
          $funcDidExecute = TRUE;
          if (!is_callable($spec['func'])) {
            trigger_error('not callable');
          }
          return $spec + ['value' => call_user_func_array($spec['func'], $args)];
        }
        return $spec;
      }, $functions);
    } while ($funcDidExecute);

    $values = array_map(function ($f) {
      return isset($f['value']) ? $f['value'] : (isset($f['default']) ? $f['default'] : NULL);
    }, $functions);

    return $values;
  }

how-to-call.php

<?php
$values = executeInOrder([
  'thing0' => [
    'deps' => [],
    'func' => function () {
      return 1;
    },
  ],
  'thing1' => [
    'deps' => ['thing0'],
    'func' => function ($thing) {
      return $thing + 1;
    },
  ],
  'thing2' => [
    'deps' => ['thing0', 'thing1'],
    'func' => function ($thing0, $thing1) {
      assert($thing0 + $thing1 === 3, '1 + 2 = 3');
      return 3;
    },
  ],
  'somethingbad'  => [
    'deps' => ['somethingbad'],
    'default' => 'answer',
  ],
  'withoutadefault' => [
    'deps' => ['somethingbad'],
  ],
  'withadefault' => [
    'deps' => ['somethingbad'],
    'default' => 12.344,
  ],
]);

The default thing should probably go. It shouldn't be used under any normal circumstance. Either that or allow some kind of pattern to return the default.