Sorts array by elements dependency [item => [items item depends on]].

Sort by dependency

<?php

function sortByDependency($input)
{
    $output = [];
    $checkList = [];
    
    $inputCount = count($input);
    
    // while not all items are resolved:
    while ($inputCount > count($output)) {
        $done = false;
        foreach ($input as $name => $dependencies) {
            if (array_key_exists($name, $checkList)) {
                // item already in resultset
                continue;
            }
            
            $resolved = true;

            foreach ($dependencies as $dependency) {
                if (!array_key_exists($dependency, $checkList)) {
                    // there is a dependency that is not met:
                    $resolved = false;
                    break;
                }
            }
            
            if ($resolved) {
                //all dependencies are met:
                $checkList[$name] = true;
                $output[] = $name;
                
                $done = true;
            }
        }
        
        if (!$done) {
            throw new Exception("Circular dependency");
        }
    }

    return $output;
}

var_dump(sortByDependency([
    'A' => ['B'],
    'B' => ['C'],
    'C' => ['D'],
    'D' => [],
]));