County.php

<?php

class County
{
  /**
   * @ORM\Id
   * @ORM\GeneratedValue(strategy="AUTO")
   * @ORM\Column(type="integer")
   */
  private $id;
  /**
   * @ORM\ManyToOne(targetEntity="State", inversedBy="counties")
   */
  private $state;
  /**
   * @return State
   */
  public function getState(): State
  {
    return $this->state;
  }
}

State.php

<?php

class State
{
  /**
   * @ORM\Id
   * @ORM\GeneratedValue(strategy="AUTO")
   * @ORM\Column(type="integer")
   */
  private $id;
  /**
   * @ORM\OneToMany(targetEntity="County", mappedBy="state")
   */
  private $counties;
  /**
   * @return County[]|Collection
   */
  public function getCounties()
  {
    return $this->counties;
  }
}