array_unique_callback.php

<?php
function array_unique_callback($array, $callback) {
    return array_intersect_key($array, array_unique(array_map($callback, $array)));
}

function book_compare_func($book) {
    return $book->get_title();
}

class Book
{
    private $title;
    private $category;
    private $date;

    function __construct($title, $category, $date) {
        $this->title    = $title;
        $this->category = $category;
        $this->date     = $date;
    }

    function get_title() {
        return $this->title;
    }
}

$a = new Book('A', 'aaa', '111');
$b = new Book('B', 'bbb', '222');
$c = new Book('C', 'ccc', '333');
$d = new Book('D', 'ddd', '444');

$book_list = array($a, $d, $a, $c, $b, $c, $c, $d);

$result = array_unique_callback($book_list, 'book_compare_func');

var_dump($result);