dep.php

<?php

class Dep {
  protected $name;

  protected $dependencies = [];

  public function __construct($name) {
    $this->name = $name;
  }

  public function add($dep) {
    $this->dependencies[] = $dep;
    return $this;
  }

  public function dependsOn($dep) {
    return in_array($dep, $this->dependencies);
  }

  public function getName() {
    return $this->name;
  }
  
  public static function compareTo($a, $b) {
    if ($b->dependsOn($a)) {
      return -1;
    }
    if ($a->dependsOn($b)) {
      return 1;
    }
    return strnatcmp($a->name, $b->name);
  }
}

$strongarm = new Dep('strongarm');
$ctools = new Dep('ctools');
$custom_module = new Dep('custom_module');
$strongarm->add($ctools);
$custom_module->add($strongarm);

$deps = [$custom_module, $strongarm, $ctools];
usort($deps, 'Dep::compareTo');
print_r(array_map(function(Dep $dep) { return $dep->getName(); }, $deps));