gistfile1.php

<?php
/**
 * So for example I have two tables "ab" and "cd". "ab" references "cd" table
 */
namespace App\Entity;

use Doctrine\ORM\Mapping as ORM;

/**
 * @ORM\Entity(repositoryClass="\App\Entity\Repository\Ab")
 * @ORM\Table(name="ab")
 */
class Ab
{
    /**
     * @ORM\Id()
     * @ORM\Column(name="ab_id", type="integer")
     * @ORM\GeneratedValue(strategy="IDENTITY")
     * @var integer
     */
    protected $abId;
    
    /**
     * @ORM\ManyToOne(targetEntity="Cd")
     * @var \App\Entity\Cd
     */
    protected $cd;
    
    ...
}

/**
 * @ORM\Entity(repositoryClass="\App\Entity\Repository\Cd")
 * @ORM\Table(name="cd")
 */
class Cd
{
    /**
     * @ORM\Id()
     * @ORM\Column(name="cd_id", type="integer")
     * @ORM\GeneratedValue(strategy="IDENTITY")
     * @var integer
     */
    protected $cdId;
    
    ...
}